What is the function of a T-square?

In Drafting, the T-Square acts as a Reference Frame for the drawing to be made. The edge that rides along the drafting table or board is a perfect 90o to the blade. The Paper is first aligned (squared up) with the T-Square's top edge. Then any lines drawn against the blade are perfectly parallel to it.

Using Plastic Triangles of various types, angled lines can be drawn that are true to the reference line of the T-Square. In this manner realistic and accurate 'mechanical" drawings can be made.

If the drawing is removed from the table it can be easily remounted using the T-Square as a Reference.