Viola
Leather mute: used the same as a typical orchestral mute, a leather mute can be a preferred option for solo works, as it dampens the harsher upper overtones, while preserving the warmth and tone of the sound. They also provide a mild volume reduction compared to a rubber or metal orchestral mute.
Practice mute: placed over the bridge on a string instrument, it largely reduces the volume. They also create a somewhat nasal tone, which can be an interesting effect for composition.
Rubber orchestral mute: dampens the sound slightly, while also dampening the high and harsh frequencies. This is a useful compositional tool, creating a more dark, soft, and velvet texture. It is placed over the bridge of the instrument, over the middle two strings.