origins of the square em

When I worked as a hand-typographer (somewhat anachronistically in the 
mid-1980s), em-blocks were square, and it was obvious why.

A square can fit into a line of type, whatever its orientation. A 
slightly off-square block would require close, eye-straining, 
examination before it could be correctly fitted into the line.

The actual width of an M is neither here nor there.

douglas