The belly is the top of the instrument containing the sound holes. It is always made of coniferous wood, usually spruce. (See here)
This wood, which is so important for the sound, is used for almost all such instruments, whether string instrument, plucked instrument or keyboard instrument. The belly of a string instrument is usually made out of two parts that have been worked out of one piece of whole wood. The bellies of violins are fitted with f-holes while c-holes are fitted into the belly of the gambas. On the inside of the string instrument, a so-called bass bar is attached under the low string for support.