I'm just about to buy some spoke nipples. I get 2 options:
1.8 x 12mm or 2 x 12mm. What does the first number refer to? Spoke thickness? I don't know this.

im guessing, but i reckon it would be the outer surface size, rather than the internal thread size.

if the internal thread is a standard, a thicker nipple is stronger, but heavier?

buy the nipples to fit your spoke key?

Careful…

The first number refers to thread diameter, you don't want to be mixing 1.8s with 2.0 spokes! If you're running double butted DTs (2.0 – 1.8 – 2.0 i think) then you'll need the 2.0 nipples, if you're running thinner lighter ones (Revolutions I think, 2.0 – 1.8 – 1.6 – 1.8_ then you'll need the 1.8 nipples.

don't quote me on the spoke dimensions though…