roado is correct, they use it because after
boiling or re setting the horns, you can gain
or lose prong measurments by tilting the tips
in or out, remember when measuring the prongs,
it is not actually the center of the rear of the
horn, but you put a strait edge along the rear of
the horn where the top edge of the prong meets, then
mark with a pencil where the strait edge touches the
backside of the horn, and start your measurment there!