The T-Plan doesn't use normal measurements-inches or mm-it uses only one measurement,
and that is the THICKNESS of the wood that you're using.
It doesn't matter whether it's 1/8", 1/4", 1/2”, 4 mm, 6 mm, 12 mm-it's still
just ONE unit, which we call “T” (thickness of wood). Since the moving parts inside
the puzzle box move a distance which equals that thickness, it makes sense to measure
the sizes of those pieces in the
same units. I make my boxes from 1/8" plywood, so I design the pieces around that
size (which I still call “T”). So a part might be 2-1/2" long by 1-1/2" high. If
you were making that box from 1/4" wood, those sizes would be 5" by 3". If you're
using 1/2" thick wood, those sizes would be 10" by 6"! If you’re using mm, it gets
even more complicated.
So you see, there would have to be a different set of plans for EVERY POSSIBLE THICKNESS
of wood available! But if you call the wood thickness “T”, you can use the SAME PLANS
for every possible thickness- the numbers used for the sizes are exactly the same
for the thickness of ALL woods.
When a piece has to move, it usually moves the thickness of the wood, to make way
for another piece to move into the space left behind.
Again, that movement is measured in units of “T”. You simply make a set of T-Gauges
from the wood you're using, and use them, AND NOT A RULER, to measure and mark the