What is a T-Plan?

A T-Plan is a way of showing the dimensions of the parts, which uses

the thickness of the wood you are using, rather than an absolute

measurement. For instance, if you're using wood which is supposed

to be 1/8" thick, and you have to cut a piece 1-1/2" long, and that

length includes the thickness of two pieces of wood, then it would

be 1-1/4" plus 1/4". If your wood was exactly 1/8" thick, then that

measurement would be correct. But if the wood was slightly thicker,

then the 1-1/2" would be wrong.

If we call the wood thickness "T", then the correct length should be

1-1/4" plus 2 times T (or simply 1-1/4"+2T).

A practical example: Cubey 3 puzzle box.

This is a box where the side panels are 2" square on the outside, but every edge is cut at 45 degrees, making the inside faces smaller. Using 1/8” wood for the inside panels, the inside faces of the side panels must be no smaller than 13 times 1/8”or 1-5/8”. No matter what the thickness of the inside wood, we call it “T”, so every inside panel starts off at 13 times T by 13 times T (or 13T x 13T). Each panel will end up smaller of course, because they all have to move a distance of 2 times T. So all the inside panels are shown against a grid, and you cut these panels to those grid shapes.

The side panels are cut so that the inside faces are 13T x 13T, no matter what the outer size of the cube will be.

The same rules applies if a piece has to move the thickness of pieces.

Instead of an absolute distance of 1/4",  3/8" or 1/2", the T-plan

would show a distance of 2T, 3T or 4T.