One common question relates to the “A” valve seen on most single Bb horns and descant horns. The central question would be “what is the purpose of placing the horn in the key of A?”
Normally valve is usually used and tuned to be used as a stopping valve, such as in this prior article:
I have noted that when I show students how a stopping valve is used on a single Bb horn or a descant horn they are often quite impressed. What is great is that you can tune it to put your stopped notes right in place, there is almost no reason to play stopped notes out of tune with a stopping valve set correctly. Why this feature is not more commonly seen on double horns probably relates to weight and complications of construction.
That all being said, there is another potential use of this valve. Let’s say you are playing a work in E with a fast technical passage, such as La Gazza Ladra overture by Rossini.
What you would do is depress the valve so that it takes your horn down a half step (from Bb to A) and then you can play in E as if you were in F, with your normal fingerings. So instead of fingering a B scale you finger a C scale but it sounds a half step low, as a B scale.
So back to the Rossini excerpt, you would with the A valve down finger the passage with the relatively simple finger pattern of a C major figure instead of having cross fingerings to deal with. This can also be accomplished on a double horn with slide extenders; Schmid horns are set up so you can accomplish this effect with the slides as constructed, in fact.
If the prospect of de-tuning your horn into an A horn does not make sense, don’t worry about it too much. It probably helps me that I studied a lot of natural horn along the way to be able to visualize playing a valved horn in a key other than nominally in F.
But the stop valve feature is still really worth having! Check it out if you never have before.