Edwin:

This doesn't answer your question, but have you tried labelling the 
squares on the TTT board using point, i.e., (x,y) coordinate pairs 
(asuming that the students ahve met graphs).

Put the origin at the center, and label top left (-1, -1), top right 
(1, -1), etc.  This gives you four-way symmetry, and a quick test for 
linearity.

	Andrew