The Normality Heuristic

I got a surprise a few months ago from one of those "word of the day" sources. Did you know what the word "normal" means? From The Free Dictionary:

normal (nôrml) adj.; 11. Conforming with, adhering to, or constituting a norm, standard, pattern, level, or type; typical.

Pretty standard, right? Ah, but looksie whence the word originates:

[Middle English, from Late Latin normlis, from Latin, made according to the square, from norma, carpenter's square. . . .]

(I emboldened.)

( So what does a carpenter's square have to do with conformity? )