Kiddicraft was a company founded by Hilary Fisher Page in 1932, in the United Kingdom. Among the many toys the company released was the "Kiddicraft Self-Locking Building Brick", originally patented in 1940. The bricks were hollow rectangular plastic bricks with 4 round studs on top. The sides of the blocks could fit between the studs of another block allowing children to build up structures such as walls.
Page patented several improvements to the basic design and the final form, developed in 1945, included slits down the side of the brick allowing flat panel-like windows or doors to be added to constructions.
After LEGO adopted the same style of bricks as Kiddicraft, they made many improvements such as adding tubes inside of the bricks to make them stick together.