A line bisecting the port runner vs. the valve stem axis.
A less acute (110° rather than 90°) angle promotes flow, but requires a taller casting, more weight, more hood clearance, longer pushrods, longer stems, etc.
In addition, the stem angle to the bore axis has an effect, and influences shrouding.
A vertical valve can't make a wedge, but it can make a good closed chamber (BMC Mini). The wedge only began high volume production in 1948 for the Cadillac engine. An existing vertical valve like the GMC, Buick L8 etc. can be improved quite a bit by closing off the chamber, but cast iron welding is not fun.
There have been closed chambers with non-vertical valves tilted in the other axis (along the crank) like the Australia Chrysler L6.
