@Intrepid,
I figured you might like that little tid-bit....but no need to feel foolish, I have met guys that were lifetime construction workers, that had never noticed them either....they prob had, but just forgot....as it is one of those things that is often seen....asked about, and whether answered or not {usually not}....forgotten.
And the knowledge...or lack thereof, of the mysterious diamonds, alone, is in no way an indicator of a persons handiness/construction skills....it is only one of many things that can be asked to measure a persons building savvy. I once had a guy, that claimed 15 yrs exp...and to talk to him, he really sounded like he knew his stuff....but it only took me a few min to figure out that he had little to no skills....rookie mistakes right off the bat. Turns out that his son...he was an older man....had been a tile setter {an apprentice} for a few years, and he had picked up on quite a few catch phrases that tile setters use.....one being..."When in doubt, grout"...so his "skills" were achieved vicariously through his sons conversations over dinner.
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I checked four of my tapes today....the two yellow Stanleys had triangles, the Kobalt had little hearts {cute huh, perhaps a play on diamonds, clubs, spades...} and a junker that I bought for my mom....JAP brand, had actual diamonds. I also checked with a carpenter that I ran into....he had a massive 35 footer, Stanley....it had triangles....and my brothers, an older silver case stanley...had diamonds. I also checked my big 100'....it has nothing on it....but those are usually pretty plain to start with. I have more, and will check them if I can find them.
I don't ever remember seeing the dots on a masons rule, which is what I use eh...half the time....cause well, I'm a mason....and we really wouldn't need anything like that...unless somebody hires me to do some repair work on a pyramid.
The carpenter I talked to today, knew they had something to do with bar joists/flat roof decking layouts...and that's the only thing that comes to mind where I have seen them used....besides the new engineered floor trusses...but you guys have mentioned those already. I've asked a few framers about using the dots to get "the" perfect roof angle, but each time that has resulted in an argument....."Oh yeah, well you just bring your buddy Pythagoras over here and I'll show him how to build a roof."
I also asked my sister in law, if she knew what the diamonds were for....she had no idea, and she has worked with my brother, for well over 20 years....I didn't have a chance to ask him, but I kinda doubt he knows either, since he is mainly a marble guy.