A building block that is releasably attachable to another block of compatible design by wedging the tongue of one block into the groove of another block, without the need for any intermediate adhesives, bonding materials or any other means of holding the blocks together. The block includes a body, an outwardly-extending tapered tongue, and two inner surfaces that form a tapered groove therebetween. The tongue and groove have substantially the same taper. The top of the tongue is wider than the bottom of the groove. The base of the tongue is wider than the opening of the groove. A tongue and groove joint is also disclosed as well as methods for forming a tongue and groove joint. A base which includes a plurality of spaced-apart tongues and grooves is provided for use in supporting a plurality of building blocks thereon.