Abstract: A toy building block (1) for stacking is provided on top (4) with one or more studs (3) and in the bottom (7) with recesses (6). The studs (4) show a toothwheel-like cross-section with rounded teeth crests (12) and rounded grooves (10) between the teeth (12) as well. The recesses (6) are provided each with a number of vertically extending lands (14) matching in shape the grooves (10). The studs easily slide in and out of the recesses (6), yet a rotational arretation of low play is obtained. Preferably, the building blocks are produced by a blowing process and may be provided with screws (21) for a safe interconnection. External slots (46) may allow the combination with panels (47) bearing images, additional functional elements etc.