Freemasonry is weird, isn't it. Its origins are shrouded in rumour and arcane mystery. One mooted theory is that the organisation came out of a fraternity consisting of a bunch of stone-masons who were not affiliated with a guild. As you were, if you were a tradesman, at the time.
So that makes them contractors, essentially. The first sub-contractors. Albeit, kinda secretive ones, with their own peculiar set of rituals and symbology.
"Peculiar?" I hear you say. Exactly. Anyone who's ever heard the bunch of tradesmen who regularly assemble down at the Occidental on a Friday afternoon will attest to the bizarre coded language in which they converse.
"Grouting" this... "9/16 locking pipe-wrench" that.
All that remains is for them to do is to bust out the secret handshakes. And anyone who's seen a gasfitter answer his cell-phone while carrying a round of 8, 9, 10 beers to the table knows to look no further.