Because all building materials change size as their temperature varies, or as they are loaded or unloaded, there becomes a need to introduce movement gaps between parts of the structures.

Materials such as concrete, and cement products shrink over time after being placed in a building, whereas clay bricks expand slowly over a long period of time.

The use of different materials and their different responses to changing loads, temperatures and moisture contents will lead to size differences between them. These differential movements must be taken into account if damage to the structure is to be avoided. This is done by the provision of control gaps or articulation joints between parts of the structure.