The present disclosure relates to building blocks for a toy building set. More particularly, the present disclosure relates to a building block configured to releasably join another building block of a similar construction. The building block comprises a hollow main body which defines a lower surface, an upper surface and walls joining the lower and the upper surfaces. At least one of the walls comprises a first wall section and a second wall section parallel to and distant from the first wall section. The building block further comprises spaced apart projections that extend from the upper surface and spaced apart recess sections disposed within the hollow main body. When two building blocks are releasably connected together, a longitudinal groove is provided between the second wall section of the lower building block and the hollow main body of the upper building block when the blocks are fully connected one to another.