understanding size of upholstery pins/tacks

[limetrees32]

It seems that one measurement is quoted but is that the diameter of the head or the length of the shaft ?
I need to tack down some rugs which are on top of carpet .

I use two different size tacks. One is thicker and has a bigger head, the other is lighter and smaller, but they both have a 10mm long shaft.

Thank you @TonTonMacoute !
I do think it's the only was I'm going to keep a rug in place .