comes to the same in the end, and the "key" must be placed on one bar
before the last three arms can be completed. The "key," moreover, must be
on one of the bars where a gap is left at the centre, and not on one
where Fig. 370 lies flush against the central arm as in Fig. 377.

UNDOING THE PUZZLE.--To take the puzzle to pieces all that is required is
to turn the "key" half round and push the other two cross bars on that
arm towards the outer point. The cross bars below may then be removed,
and the whole structure falls to pieces.

THE DOUBLE DOVETAIL PUZZLE (Fig. 379) consists of two pieces of wood
(usually one dark and the other light) which, upon examination, appear to
be dovetailed together from each face. This interlocking arrangement is
obviously impossible, and the solution of the puzzle is only apparent on
examining Fig. 380, where it will be seen that the joint fits together
diagonally.

At Fig. 381 are given the diagrams for setting out. Draw the outline of
the elevation, plan and end view. The end view in the first instance is
indicated by 3, 4, 5 and 6, and it measures 1-7/8 ins. square. A
1-7/8-ins. square is simply used because 2-ins. wood generally finishes
this size after it is planed up. Set out a square (A, B, C, D) which
stands corner-ways in the larger square (3, 4, 5, 6). Project the lines D
A and C B upwards as at 1, and on to this drawing (1), set out the
dovetail according to your own idea of length, width and bevel. Project
the four points of your dovetail downwards into the end view, and where
these lines cut A, B, and D, C draw them downwards and rebate them into
your original plan. This will give the true shape of the two dovetails
and it is to this shape that you will cut your joint.

The joint is in due course glued up, and next day you will plane and
waste off the four corners of your model. The end view shows one corner
shaded D, 3, A; this and the other three corners are wasted away. The
result is that the dovetails are thrown into a plane different from that
in which they were made, showing as Fig. 379.

[Illustration: Fig. 379.--Double Dovetail Puzzle.]

[Illustration: Fig. 380.--The Two Parts Separated.]

[Illustration: Fig. 381.--Elevation, Plan and End View, showing how the
Puzzle Joint may be correctly Set Out.]

[Illustration: Fig. 382.--Dovetail Puzzle. The Finished Joint.]

[Illustration: Fig. 383.--Sketch of Dovetail Piece.]

(Note that dovetail is cut on slant, the