"Highland Pairos" wrote in message
...
I assumed that it was necessary for setting a triangle 90 deg to a t-square
that is set along the edge.

Is this not so?

You square the drawing paper to the T-Square. Then you set the triangle on
the top edge of the T-square. Some drawing desks have this ledge to keep
the drawing board from sliding off of the desk it the desk top angle is too
steep.