Bricks became one of the most common building materials. In most cities, nearly all the houses are built of bricks because they are long lasting, they do not need repairing very often. Brick houses are generally made strong by having all the main walls two bricks thick. Two walls are built side by side which are "bonded" (or joined to each other) by having some bricks set so they make a bridge between the two layers. The pattern of the bonded bricks can be seen from the outside. The two main patterns are called "Flemish Bond" and "British Bond".