A daughterboard, daughtercard, mezzanine board or piggyback board is a circuit board meant to be an extension or "daughter" of a motherboard (or 'mainboard'), or occasionally of another card. In particular, daughterboards often have plugs, sockets, pins, connectors, or other attachments for other boards, which is what differentiates them from standard expansion boards such as for PCI or ISA, which are usually called expansion cards.