So, imagine you have a regular-sized violin, but it's much, much bigger - like tall as you are! That's a bass violin, also called a double bass or upright bass. It's the biggest instrument in the violin family and makes a super deep sound.
It looks kind of like a big cello or a giant guitar, with a wooden body and a long, skinny neck. To play it, you have to stand up and hold it against your body, kind of like a hug. You use your left hand to press down on the strings and your right hand to pluck or bow them.
Because it's so big, it's not as easy to play as a regular-sized violin. You have to be pretty strong to hold it up and move your fingers around the long neck. But when you get the hang of it, you can play really cool music that makes people want to dance or feel sad or happy or all of those things at once!