Well usually there's these two tubes called the down tube and the top tube - you put it on the end of those... Being a little more serious, the head tube just ends up where the rest of the geo tells it to. If your wheelbase is X, back end A, head angle B, fork length C and the BB rise Y, then the head tube will just end up in the right place to conform to all these other dimensions. That make sense? Of course you could make head tube placement one of the things you base the rest of the bike's geo around, but for all other frames at the moment it seems to be the other way around...