How are drums classified by the shape and construction of their shells?

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Flemish rommelpot friction drum — percussion instrument Flemish *rommelpot* friction drum; in the Musical Instruments Museum, Brussels.

Drums are classified as membranophones within the percussion instrument family, defined by a shell—typically wood, metal, or pottery—covered by a vibrating membrane. Their classification based on shell construction and shape includes:1

Tubular drums: These assume various shapes, including barrel, hourglass, and goblet forms. They are considered shallow if their height is less than their diameter.1
Frame drums: These are so shallow that the shell cannot act as a resonator for the sound. Their shapes vary widely, including round, square, and octagonal designs.1
Kettledrums: These are bowl-shaped instruments, historically made of clay or wood and later constructed from metal.1

Shells may be covered at one or both ends by a membrane, which is secured using various methods such as pegs, nails, glue, or lacing. While the term "drum" generally refers to membrane-covered instruments, it is sometimes applied to non-membrane struck instruments like slit drums or steel drums.1