Manual of Purpose-Made Woodworking Joinery

Frame joints 21

generations of master craftsmen, have been eroded
to some extent in the second half of the last century.
Traditionally, a through- tenon, reduced in width to
allow safe wedging near the corner (and good contain-
ment of the tenon), used to be reduced by a half (not a
third, mentioned above) of the tenon’s available width
(after reduction for panel- grooves or rebates, etc), as
illustrated in Figure 3.3(b). By this half- and- half rule,
the structural conflict created at corners of frames, by
the tenon being strong in full width and the mortise
being weak, was resolved by equal proportioning of
the tenon giving up half its width and the mortise
losing half its length.

Haunched areas and tenon divisions

Figures 3.4(a) and (b): When a portion of a tenon’s
width is removed, as explained above, it should not
be removed entirely. A small stub, usually equal to the
thickness of the tenon, is left as a projection from the
shoulders. This projection is called a haunch and the
groove to be cut in the stile to receive it is extended
from the outer- end of the mortise and is called a
haunching or haunching groove. The tongue- and- groove
effect of the haunch- and- groove adds strength to the
joint, but it also helps to restrain the tenoned rails
from any future cupping or twisting movement. Note
that if a stile is grooved to hold panels, the groove is
used as the haunching and the haunch should fill the
groove. A panel groove (if taken along the entire edge
of stiles – which is usual) should not be wider than the
tenon- and- haunch thickness, but, of course, it can be
narrower.
Double tenons on the bottom rails of doors are
usually found to conform to the half- and- half rule
mentioned above, but – because of the necessity for
a haunch at the bottom – the rule (still conforming
to half the total tenon- area, half the total haunch-
area) has to be in the form of four quarter- divisions:
two tenons and two haunched areas, as illustrated in
Figure 3.4(a).
Tenons for traditional, wide middle rails of doors
(originally referred to as lock rails), unlike the division
into thirds nowadays, also met the half- and- half rule,
but in the form of two quarter- width tenons and a
middle haunch area between the tenons, equal to half
the tenon’s total width, as illustrated at Figure 3.4(b)
above. Of course, this arrangement of tenons also
provided the mortises in the stiles with an adequate
mid- area of long- grain for the subsequent insertion
of a deep mortise lock and door- knob furniture of
yesteryear.

Tenon- division conclusions

Figure 3.5: Finally, with reference to tenon- division

for entrance doors (room- doors usually being mass

produced nowadays), I have also seen – in text books

and joinery shops – fifths used for the division of

the available width of top- rail tenons, in the form of

two- fifths haunch and three- fifths tenon. This seems to

be a fair compromise between the half- and- half and

the one- third/two- thirds techniques – but, math-

ematically, the result is so close to the minimum of

38mm short- grain rule, that the latter could be used

without any dividing technique! My maths – related

to the scaled drawing at Figure 3.5 below – were

based on a typical, traditional top- rail width of

115mm sawn, reduced to 108mm prepared, minus

a panel groove of 12mm, resulting in an available

tenon width (ATW ) of 96mm. Therefore 96 ÷ 2 =

48mm haunch (half and half rule); 96 ÷ 3 = 32mm

haunch (one third/two thirds rule); 96 ÷ 5 = 19.2 ×

2 = 38.4mm haunch (two fifths/three fifths rule). Note

that this last calculation is very close to the minimum

38mm rule.

Mixing the various rules, I usually favour the

fifths rule (or 38mm minimum rule) for top rails of

entrance doors; the thirds rule for wide middle rails

(unless a deep mortise lock is to be fitted between

—^1

4

—^1

4

—^1

4

—^1

2

—^1

4

—^1

4

—^1

4

(a) (b)

Figures 3.4 (a) and (b) Division of tenons for bottom- and

middle- rails.

—^1

2

—^1

2

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—^253

—^3

5

—^2

3

A

T

W

TOP

RAIL

HAUNCH

38 mm

SHOULDER

GROOVE, REBATE OR STUCK-MOULD PROVISION

TENON

Figure 3.5 Optional divisions of the ‘available tenon

width’ (ATW).