A type of ionic crystal structure in which the cations have a face-centred cubic arrangement, with anions occupying all the octahedral holes. It can equally be described as a fcc array of anions with cations in the octahedral holes. Each type of ion has a coordination number of 6. Examples of compounds that have the structure are NaCl, KBr, AgCl, AgBr, HgO, CaO, FeO, NiO, and SnAs.
http://cst-www.nrl.navy.mil/lattice/struk/b1.html An interactive version of the structure