Centre of the chainring from the centre of the frame. 42 is the track standard so that's the way track hubs are set up. But then they're also 120mm wide, so you can compensate for the difference when you space your hub up from 120. There is a boatload of possible solutions. There are 130 hubs as well, which would be easier to install but it'd be trickier to play w/chainline. Paul hubs have a wider chainline to begin with.