a sash horn could be called a "stop" to prevent the sash from opening too far...

What you carry when wearing a tuxedo and want the bartender's attention!

I have actually heard of this term but don't want to blow it for others who will likely have wittier responses.  

Looks like Barbara gave you a great answer.

Looks like someone left the window open ;)

Helps keep the boogeyman and the thieves out of the house!

Tony and Suzanne Marriott, Associate Brokers - looks like I will have to stop by daily and learn something new. Thanks for your Questions - and now, answers.

window frame corner or something....

Sounds like a fancy decorative horn that is not unusable

This has to be something to keep a sash back. 

A sash is section of a window so suppose back in the day they had a horn to keep it open.

It obviously has something to do with windows.

What Barbara Todaro said.

I believe it has nothing to do with music?

I'm going to say something regarding a 'winderrrr'.

Tony and Suzanne Marriott, Associate Brokers Might have something to do with windows.

Something to do with windows.

Now that's one feature that has saved my ambidextrous digits on more than one occasion.

I like Raymond Camp's answer. One way to get attention in a tight joint!