Every time I do this I think there might be a "better" way. Draw a line, any angle. Draw a second line perpendicular to the first line where the second line starts at the midpoint of the first line. This should create a "T" of sorts, lengths are not important for the excercise. I've always drawn the second line by starting a distance away from first, then back, perpendicular snap to the first line, move to midpoint. I'm not looking for a lisp routine or coding, just curious if there's a..different way..