The building block (1) is quadratic in section, and symmetrically arranged around a plane (M) cutting through the centre of the block. The bonding faces (2) have a series of projections (5) and recesses (6) for bonding blocks. The bonding face is sloped (7)in for the front faces (4). A recess (8) is formed in at least one of the abutting faces (3). The block may have a channel (10) running through it. The recess (8) may be provided by an inclined shoulder (9).