Have you heard of a symphony orchestra? It is a large group of musicians who play music together. Some musicians in this group play string instruments. String instruments are made of wood and strings.
The violin is the smallest string instrument. It is about two feet long. The violin also has the highest sound.
The viola is a little bigger than the violin with thicker strings. So the viola has a lower sound than the violin.
The cello is even larger with even thicker strings. It is about four feet long.
The double bass is the largest string instrument in the orchestra. It is more than six feet long. The double bass makes the lowest sound in this group.