Abstract: This invention relates to building blocks, and more particularly to building blocks having greater strength in the individual blocks, and which are capable of embodying greater strength between side edges of the blocks. The inside vertical edges of the blocks are slotted with rounded end portions to receive on the inside portions of the wall to provide the contoured side ends of the cross members to provide a strong rigid support, and firm connection between the side members and the connection between the cross pieces. In this way the outer edges of the wall members can be formed smooth with no interruptions except for the side edges of the blocks.