This is a Mach One shoulder rest for a small viola. Size: Medium. I used it for a while, but it often slipped off because it was too wide, so I replaced it with a Small. It shows signs of use, but there are no problems with normal use. I felt it was wide for a 42cm instrument. It might be just right for people who usually put the shoulder rest on deeply. - Model Name: Mach One - Size: Medium - Material: Wood - Color: Brown - Features: Non-slip Thank you for looking.