It's very possible that you have not heard of the clever German musician and composer Louis Spohr. I admit that I hadn't, and his name and accomplishments only recently became known to me.
As a composer, Louis is not particularly well known now, even though he was pretty well known as a performer and composer when he was doing both. But almost every performance of classical music today owes him for two significant contributions.
One, he invented the chin rest for the violin. Virtually every violonist uses it now, and Louis used it when performing back in his day. It made it easier for the musicians to play in increasingly longer works, and also made it easier to hold the violin or viola when playing, so that it wouldn't slip out and get dropped.
The picture below shows a 19th century chin rest. They weren't as smooth and form-fitting as the modern versions.
The other thing Louis invented was the conductor's mark in the score. This allowed the conductor to say to the orchestra he's rehearsing, "Let's start five bars before E", rather than have to count stanzas from the beginning.
A-D are the rehearsal marks (much more closely spaced in this example than in an actual score).
So Louis was both a violinist and composer and conductor - he was also one of the first conductors to use a baton. Next time you hear a 19th-century symphony, think of Louis.