I learned out how to climb multi-pitch at Tahquitz, where the American grading system was first established. 5.9+ in that area is a terrifying grade. American grades were originally a closed system that topped out at 5.9. For a time climbers were climbing harder and harder routes, but rating them all 5.9. The system eventually got switched to be open ended, but grades are sticky and it definitely helps to know when a route was established in addition to what the guide book rating is.
Yes it was the same for the British adjectival trad system, ES used to be the top grade (after HVS) and this grade is now divided into an open ended scale E1-E11... but I think this probably caused some hard climbs to be categorised as HVS in an attempt to differentiate from the _really_ hard climbs in ES at the time. It's worth checking the FA date around this grade in old crags for this reason, but even being aware in my case - it was surprising just how much harder they can be.
