Sheet music is best defined by physical format— it occurs as single sheets printed on one or both sides, folios (one sheet folded in half to form four pages), folios with a loose half-sheet inserted to yield six pages, double-folios (an inner folio inserted within the fold of an outer folio to make eight pages) and double-folios with a loose half-sheet inserted within the fold of an inner folio to produce 10 pages. The content of sheet music varies widely— it can be either popular or art music, and while songs and solo piano music are the most commonly represented genres, sacred and secular choral music, dance music, and other genres also appear in this format.