Abstract: A building block comprising a panel member having a forward panel surface and a rearward panel surface, and a peripheral skirt extending rearward from the rearward panel surface and defining a receptacle; wherein an interconnection means adapted for making sidewise locked interconnection with an adjacent building block having a complementary or counterpart interconnection means is formed on the peripheral shirt, the interconnection means being a female connection member or a male connection member complementary to the female member; wherein the female member is shaped or adapted to permit a male connection member of the adjacent building block to enter into sidewise locked interconnection by entering at the free end of the peripheral skirt which is distal from the panel member, to move towards the forward panel surface, and to stop before reaching the forward panel surface.