Handlebar stem measurement

dombo6

Can anyone advise how this is measured? eg a 100mm stem, is that referring to the physical length of the whole stem, or the distance from the middle of the h/bar hole to the middle of the steerer hole?
Many thanks.

sandbag

It's from center of the headset to the centre of the stem.

dombo6

Thanks!

sandbag

I meant to say, center of the headset to the centre of the handlebars. But it means the same thing.