Mortise and Tenon: A Primer - Woodworker's Journal

32 

Mortise and Tenon: A Primer 

Joining long grain to end grain with mortise 

and tenon — the king of rail joints 

By Ian Kirby 

A 

mortise and tenon joint is 

nothing more than a square 

peg in a square hole. The joint 

is either stopped or through. The 

through joint is usually wedged, 

which makes it effectively a dovetail 

joint. Chances are there isn’t a house 

in the U.S.A. that doesn’t have 

examples of the joint somewhere in 

its furniture or built-in cabinets. 

In this article I’ll describe the 

three main ways the joint is used. 

Since the geometry of any design 

follows a certain set of conditions, 

it’s vital to understand that aspect of 

the joint. I’ll also explain how to 

design for strength. 

Two-dimensional frames 

(doors, mirrors, wall panels) 

In a subsequent article I’ll put 

theory into practice by looking at 

ways the joint can be cut and 

describe, through the making of 

a simple table, what you must do 

before and after cutting the joint. 

Although the basic joint is simple, 

its design details vary in complexity 

and sophistication according to 

the following circumstances: Do 

the two or three pieces being 

joined form the corner of a twodimensional 

frame, as in a door, or 

a three-dimensional frame, as in 

a table? Or, do they form a T-shape, 

as in a center rail? And finally, is the 

joint through or stopped? 

Three-dimensional frames 

(tables, chairs, beds) 

Although these pieces of furniture were built by three different craftsmen and 

represent vastly differing styles, they share the mortise and tenon as their foundational joint. 

Over the years the mortise and 

tenon assumed a variety of design 

forms for a variety of reasons. 

I’ve illustrated some of them along 

with brief comments. Regardless of 

their design peculiarities, to work 

correctly each must follow the 

general design characteristics of the 

more common forms. 

In many joinery situations where 

a mortise and tenon would have 

been the traditional solution, two 

recent alternatives have proved 

their worth and may be appropriate. 

They are plate joints and pocket 

screws inserted with the aid of 

oriented holes. 

Carcasses 

(bookshelves, etc.) 

December 2001 Woodworker’s Journal

Anatomy of Parts 

A key step to understanding 

mortise and tenon joinery is 

knowing the parts, many of which 

are derived from human anatomy. 

The specific name of the 

mortise/tenon piece depends upon 

its function and orientation. The 

mortise piece is usually the upright 

member, such as a stile, leg, or 

post. The tenon piece is usually 

horizontal, such as a rail, an apron, 

a stretcher, or a shelf. 

Dimensions of Parts 

A = Tenon length = Mortise depth 

On a stopped joint leave a gap 

between the end of the tenon and 

the bottom of the mortise. The two 

parts don’t need to meet since the 

end grain of the tenon isn’t a gluing 

surface, and it’s easier to make if 

you leave a gap of about 1/16". 

B = Tenon width = Mortise length 

Make the tenon width to exact fit. If 

it’s too narrow, the tenon may float 

and position the rail in the wrong 

place. For the same reason, don’t 

crush the the edges of the tenon 

during assembly thinking you are 

making the joint tight. More likely 

you are causing misalignment. The 

end walls of the mortise are end 

grain and therefore not suitable 

gluing surfaces. 

Woodworker’s Journal December 2001 

Mortise 

piece 

Stile 

End 

wall 

Bottom 

Cheeks 

B 

A 

C 

Defining the parts 

Side wall 

B 

C 

End 

C = Tenon thickness = Mortise width 

Make the tenon width to exact fit. If 

it’s too thin, it will likely float and 

position the rail in the wrong place. 

You should be able to “push fit” 

the parts together. A “clamp fit” 

with squeaky resistance is too tight. 

Applying glue will swell an easy-fit 

joint much tighter. A too-tight joint 

might split when glue is added. 

Geometry of Parts 

Accurate joinery, whether cut by 

hand or machine, is defined by 

accurate geometry. 

Some authors recommend undercutting 

the shoulder to ensure 

a tight-fitting joint. My view is — 

don’t. First, what is undercut? 85°? 

70°? Second, how do you measure 

it? Third, if you can undercut 

consistently, its stands to reason 

you can also cut square. 

Shoulders square to face 

side and other faces 

Faces aligned with stock, 

with no twist 

A = Tenon length = Mortise depth 

B = Tenon width = Mortise length 

C = Tenon thickness = Mortise width 

A 

Face 

Edge 

Cosmetic 

shoulder 

Tenon 

piece 

Rail 

Structural 

shoulder 

The undercut shoulder presents 

an edge to the stile instead of a flat 

face. Clamp pressure easily pushes 

the edge into the stile, distorts the 

wood, and changes the between-theshoulders 

distance. Most important, 

you also lose the mechanical 

efficiency of a dead stop. 

Mortise and tenon joinery is 

subjected to two types of stress. 

One type arises from use, as when 

we lean back in a chair or push 

a table to a new location. The other 

type arises from wood movement. 

Wood shrinks and expands across 

the grain in response to changing 

humidity but not along its length. 

To cope with both types of stress, 

we design the joint to be as strong 

as possible. Since the configuration 

and section of the parts being 

joined are usually unique, we must 

design for the job at hand and that 

design is based on experience 

about what has worked before. 

End walls square to the 

face side and face edge 

Side walls parallel to each 

other and to face side

34 

Designing for strength 

Shoulders 

All tenons require shoulders. There 

are two types. Structural shoulders 

resist bending stress. Cosmetic 

shoulders hide the joint if the rail 

shrinks and they hide the gap 

presented by a damaged mortise 

end or a mortise made slightly 

long. Cosmetic shoulders need be 

only 1/8" to be effective. 

A tenon without a shoulder on 

one side is called a barefaced 

mortise and tenon. To keep it 

aligned at the glue-and-clamp stage 

requires purpose-made clamping 

blocks. I don’t recommend this 

configuration. If at all possible, 

reduce the thickness of the tenon to 

get a shoulder on each side. 

No shoulder 

on this face 

A tenon without a shoulder on one face of the 

stock, called a barefaced mortise and tenon, 

is not recommended. Try reducing the tenon 

thickness to get a shoulder on the bare side. 

Tenon Length, Mortise Depth 

If the tenon is too long, the joint is 

under greater stress due to wood 

movement. Too short, and it may 

fail due to too little glue surface. My 

compromise is to mortise about 

halfway into the stile. In narrow 

stock, go a bit more than halfway. 

Horn 

1/16" 

gap 

Tenon is 

too thin 

for 

typical use 

1 ⁄3 1 ⁄3 

1 ⁄3 

When a mortise is placed near the end of a stile, 

an extra inch of so of material is left to extend 

past the joint. This horn will be cut off later. 

Tenon Width, Mortise Length 

If a mortise extends too near to 

the end of the stile of a corner 

joint (often, exactly the spot 

where you simply need a joint to 

go), the stile may split. Therefore, 

make the tenon shoulder nearest 

the end of the stile one-quarter 

the total width of the 

tenon. To keep from 

splitting the mortise, 

an extra inch or so 

(called the horn) is 

left on the stile. Once 

the glue is cured and 

the job is out of 

clamps, the horn is 

sawn off. 

When the mortise is 

not at the end of the 

stile, make the tenon 

the full width of the 

rail, less 1/8" at each 

edge for the cosmetic 

shoulder. 

Preferred 

machining 

of the 

joint 

3 

⁄16 

" 3 

⁄8 

" 3 " 

⁄16 

Make the tenon 

shoulder nearest 

the end of the stile 

1/4th the total 

width of the tenon. 

MORTISE & TENON RULES 

1. Hidden mortises should penetrate 

about halfway into a stile. 

2. Create Balanced joints. 

3. Structural shoulders resist stress. 

4. Cosmetic shoulders hide the joint. 

5. At the end of a stile, leave a horn. 

(to resist splitting) 

6. Don’t undercut. 

When joining two pieces of the same thickness, 

balancing the joint for strength amounts to 

making the thickness of the two cheeks added 

together equal the thickness of the tenon. 

Tenon Thickness, Mortise Width 

When joining two pieces of wood of 

the same thickness, divide the 

thickness roughly in half to create 

a balanced joint. For example, if the 

wood is 3/4" thick, make the tenon 

3/8" thick and each mortise cheek 

3/16" thick. Since each piece has an 

equal amount of tissue, each is 

equally able to resist stress and the 

joint is balanced. 

Loose-tenon Joint 

A slot mortiser is a machine that 

cuts a mortise into both rail and 

stile. The two parts are connected 

with a third piece made to fit the 

slot, much like a biscuit joiner. This 

joint is called a loose mortise and 

tenon. You can make the joint using 

a plunge router and a single bit, 

although you will normally need 

different jigs to manage the two 

parts. Once glued, the joint is just 

as strong as a conventional mortise 

and tenon. 

Floating Tenon 

Loose tenon joints are the 

inspiration for latter-day biscuit 

joiners. Once glued in place they 

are as strong as a traditional 

mortise and tenon joint. 


Two-dimensional frames 

Mortise 

& Tenon 

Variations 

Nearly as soon 

as there were 

woodworkers, the mortise 

and tenon joint was 

created — an efficient 

and elegant way to join 

solid wood. Variety being 

the spice of life and the 

essence of creativity, 

many kinds of mortise 

and tenon joints became 

common. Due to advances 

in machinery, adhesives, 

and techniques, the 

mortise and tenon 

variations illustrated here 

are seldom used today. 

Long 

and short 

shoulder 

mortise 

and tenon 

Frame and Panel 

Two-dimensional frames with a solid panel 

trapped in a groove make a classic frame 

and panel. This ingenious construction is 

not only the foundation of furniture 

making, it remains the only method we 

have for making a dimensionally stable 

frame from dimensionally unstable solid 

wood. The traditional example is, of course, 

the frame and panel door. Although with 

modern material this door is no longer the 

most efficient design available, tradition 

and a sense of history continue to drive the 

popularity of this beautiful cabinet staple. 

Used in multiples, we can make larger 

frames, which, joined at the corners, make 

storage chests and cabinets. 

Haunch 

mortise and 

tenon 

1 

⁄4 

A stile turned 90˚ to 

the rail provides the 

greatest resistance to 

stress. Typically the 

through tenon in that 

situation will be 1/4th 

the width of the rail. 

Fox tail wedge 


tenon 

As the rail is driven 

home, the wedges 

expand the tails. 

In the days of hand tool 

woodworking, grooves 

were made using a plow 

plane. Because you can’t 

make a stopped groove 

with a plow plane, the 

haunch was left on the 

tenon to fill the groove. 

If the frame was to be 

fitted with a glass panel 

sitting in a rabbet, then 

the long and short joint 

was used. 

The author has 

made many fox 

tail joints but 

never used one in 

furniture! You only 

get one shot at 

glue-up. It’s 

a clever concept, 

but there’s no 

place in furniture 

where it’s really 

necessary. 


7. The completed joint 

should slide together with 

moderate hand pressure. 

8. Accurate geometry is the 

key to effective joinery. 

9. With stopped tenons, 

always cut them 1/16" 

short of the end wall. 

10. Use a sufficient amount 

of glue on the joints (but 

avoid over gluing.) 

A middle rail A top rail 

Balanced 

cosmetic 

shoulders 

Mortise and tenons and the flat 

panels easily created by them have 

long been the building blocks of solid 

wood construction. Adjust the 

position of the tenon at the end of 

the rail to logically suit the location 

of the rail in the panel. 

Sloping 

haunch 

mortise 


Pegged 

mortise 


By slightly offsetting 

the through tenon, you 

can reduce your 

chances of splitting 

the stile and add 

stability to the joint. 

Shoulder 

is offset 

from top 

of joint 

to avoid 

splitting. 

On a threedimensional 

frame, 

there is no need for 

a groove. The notion 

of a haunch, 

however, remained. 

A sloping haunch 

satisfies the 

psychological need 

for a haunch and 

the visual need for 

a flush shoulder line 

at the top edge. 

Draw pegging 

is a technique 

derived from 

timber frame 

construction. It’s 

unnecessary with 

modern glues and 

correctly made 

joints, but if you 

like the look, add 

the pegs after 

the joint is made. 

Woodworker’s Journal December 2001 35

Tables, chairs 

and other 

pieces of 

furniture are 

created from 


tenon joints 

constructed 

in three 

dimensions. 

36 

Three-dimensional frames are 

used to make table bases, chairs, 

and the skeletons of large cases. 

Three members generally connect, 

for example, a table leg and two rails 

or aprons. In most situations the 

vertical leg is more or less square in 

section, while the incoming rails are 

flat. Factors affecting the design of 

the joint include the width and 

thickness of the rails, the exact 

location of the rails on the faces of 

the leg, and complications caused 

by drawers or doors. 

Underframes impart stability 

to a piece of furniture. On this one 

the design not only includes twin 

tenons, but wedged tenons as well. 

Three-dimensional frames 

Inset Rails 

Flush Rails 

Centered 

Rails 

The illustrations above show 

the rail in three different place on 

the leg. Once the joint is glued, the 

“flush rail” must be made dead 

flush by planing — a manufacturing 

detail unnecessary with the inset 

rail. However, the strongest reason 

to inset the rail is visual: the change 

in level, along with the associated 

highlights and shadows, emphasizes 

the vertical element of the leg. 

Leg 

Stretcher 

Crossbar 

With three 

dimensional frame 

construction, 

design 

considerations 

become almost 

infinitely variable. 

Regardless of how 

the joint is 

employed to 

create a pleasing 

appearance, the 

rules of mortise 

and tenon joinery 

must be observed. 

Note that the thickness of the 

tenon is increased as it is inset while 

the gluing area decreases. In each 

case the tenon is mitered in the 

middle of the joint. Provided the 

tenons don’t touch, you can make 

the gap as small as you like. The 

1/8" inside shoulder on the flush 

and inset rail is enough to do the job. 

Underframes 

Underframes impart stability to 

a piece of furniture by bracing the 

structure. They often do double 

duty as supports for shelves. 

Where two underframe members 

meet at a T-shaped junction, you 

can increase the gluing area by 

using twin tenons. If the joint is 

made through and wedged, it offers 

an attractive design detail. 

Twin Tenons 

When the rail is thicker than 1", you 

can increase the gluing area by 

making twin tenons side by side. 

The total thickness of the two 

tenons is about the same as for 

a single tenon, but the doubled 

gluing area makes for a potentially 

stronger joint. The figure below 

shows typical proportions: the 

minimum thickness of each tenon 

is about 3/16", and the space 

between should be about equal the 

tenon thickness. Twin tenons are 

often used this way in chair making 

where a back rail joins a legs. 

Thicker stock offers the possibility of creating 

twin tenons. The key advantage to this 

modification is you nearly double the glue area. 


Short twin tenons also occur in 

carcass sides, where a partition ties 

the case together and also acts as 

a shelf. Make the housing (also 

known as a dado) about 1/8" deep 

between the mortises. A housing 

between tenons not only makes the 

shelf able to bear a heavy load, but 

it also flattens any cup in the two 

boards. Because the tenons would 

otherwise be very short, they are 

usually made through and wedged. 

Because of the 1/8" deep housing, 

a 1/2" thick tenon is plenty big. 

Stop 

short by 

1/4". 

Gradient 

1 = 8 

Wedge is 1/8" 

longer than 

the kerf. 

A through-wedged mortise and tenon demands 

precision joinery. The wedge must thrust the 

stock against the end grain of the mortise. 

Place it the other way and you’ll split the wood. 

Gluing and Clamping 

A simple yet elegant idea like the 

mortise and tenon joint can go 

wrong at the last moment. In a later 

article, I’ll go through the basics of 

gluing and clamping. But for now 

I must simply say that incorrectly 

applied clamps and their critically 

important clamping blocks, or 

cauls, will ruin the best crafted 

joints in short order. So do not give 

short shrift to this last step in 

making a joint. 

With these few concepts in mind, 

it’s time to go to your shop and join 

some wood. Good cutting! 

Carcasses 


11. Don’t think you can make accurate 

mortises by drilling the waste away 

and “cleaning up” with a chisel. If you 

don’t form your mortises entirely by 

hand, try these easy to follow 

steps. 

Ian’s recommended process: 

a. After marking the joint, use an 

appropriate drill bit to 

carefully remove most of 

the waste. 

b. Use a plunge router and 

a router jig to further 

machine the mortise. 

This will give you parallel 

walls square to your 

work piece. 

c. Pare the corners 

square and true with 

a sharp chisel. 

One place where through mortise and 

tenons, and particularly twin tenons, occur 

regularly is in projects such as a bookcase. 

The through tenons are both structurally 

integral and visually beautiful. An additional 

joinery technique that the author regularly 

employs is a conjoined housing (or a dado 

here in America). The 1/8" deep dado 

shown between the mortises increases 

the weight-bearing capabilities of the shelf 

and flattens any cupping in the shelf and 

upright pieces. It also presents a clean 

and subtle joint line. 

In this example, the author recommends a shallow 

dado stretching between the through mortises. 

Woodworker’s Journal December 2001 37

Mortise and Tenon: A Primer - Woodworker's Journal

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