This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
A plain T-square, with a parallel blade fastened on the side of the head, but not imbedded into it, is the best; in this way set squares can be passed over the head of a T-square in working at the edges of the drawing. It is strange that a drawing square should ever have been made in any other manner than this, and still more strange, that people will use squares that do not allow the set squares to pass over the heads and come near to the edge of the board. A bevel square is often convenient, but should be an independent one; a T-square that has a movable blade is not suitable for general use. Combinations in drawing instruments, no matter what their character, should be avoided. For set squares, or triangles, as they are sometimes called, no material is so good as ebonite; such squares are hard, smooth, impervious to moisture, and contrast with the paper in colour; besides, they wear longer than those made of wood. For instruments, it is best to avoid everything of an elaborate or fancy kind.
Procure only such instruments at first as are really required, of the best quality, and then add others as necessity may demand; in this way, experience will often suggest modifications of size or arrangement that will add to the convenience of a set.