Seatpost diameter

[Levo-Lon]

Thomson engrave them where they can be read, I think.

indeed but you pay for it

 

[HLaB]

I'm not sure of the bike's I had as a kid but the majority of bikes I have had as an adult have either been 27.2mm or 31.6mm. I did have one bike where I had to shim a 27.2mm post with a coke can to get it to fit in the frame.

 

[rogerzilla]

I have, on various bikes: 26.6, 26.8, 27.0, 27.2, 28.6, 30.9 (in a 36.9 frame, shimmed), 31.6 and 31.8! But, to be fair, 27.2 is the most common.