I see all these suggestions online for how to measure one's sitbone width at home, but I've never/rarely seen this one detail clarified: does one measure inside-inside, center-center, or outside-outside?

Specifically, I'm interested in measuring for leather saddles...I have good experiences with plastic-based saddles with a listed width of 143mm, and a leather saddle listed at 160mm. Another leather saddle listed at 146mm ended up being too narrow (I found myself sitting on the crescent part of the frame), but I'm wondering how to determine the narrowest I can go with a leather saddle. I know I need to account for a bit of extra width so that I'm not sitting on the frame, of course...

Ok, now I'm confused...decided to try measuring directly with a digital caliper, and if I've done so correctly, I'm getting about 120mm...so if you go with the usual recommendation of adding an extra centimeter on each side for a leather saddle, my 146mm Gilles Berthoud saddle should theoretically work. Maybe this was actually an issue relating more to saddle setback than width (the bike I had this saddle on has a 74 degree seat tube angle)...I'm gonna try taking some more direct measurements from the saddle itself.

Turns out that the two Berthoud saddles measure the same on the inside of the cantle/crescent part of the frame: 104mm. However, I found that on both of them, I sit just in front of the cantle, but on the overall-narrower one, there's not enough material to support me without hitting the "corners" of that part of the frame, and setback doesn't seem to help. Neither does a more aggressive position with the bars well below the saddle. I guess I've confirmed my suspicions of this saddle just not working for me!

Different saddles of the same supposed width will fit differently. It's way more complicated than get the width right and the saddle will fit. You just have to ride them and see if they work. No amount of measurement will tell you that in advance. That said, it doesn't hurt to start with a saddle that's theoretically the right width. Doesn't mean it'll work, though.

Also leather saddles work for some people in a particular range of position, but they don't work for all people or for any person without regard to position. Composite saddles are interesting in that they can be made in any shape, just like people.