There are a lot of different reasons to add a key guard.
Players with vintage curved sopranos commonly find that they need an Eb guard to keep from bending that key and catching it on clothing.
This guard is to keep clothing out of the low Eb. Specifically for this horn, the Eb guard was modified to keep Chris’ tuxedo jacket out from under it.
Guards can be added to help with pitch
We decided to make the Low C guard taller on all of our Aristocrat Altos. No one likes a flat D.
Revamped guard installed. It’s taller and not connected to the tone hole.
Guards installed over the high E and F to protect the keys around the crook.