Lego pieces of all varieties constitute a universal system. Despite variations in the design and the purposes of individual pieces over the years, each remains compatible in some way with existing pieces. Lego bricks from 1958 still interlock with those made presently, and Lego sets for young children are compatible with those made for teenagers. Six bricks of 2 × 4 studs can be combined in 915,103,765 ways.

Since 1963, Lego pieces have been manufactured from ABS plastic. As of September 2008, Lego engineers use the NX CAD/CAM/CAE PLM software suite to model the elements. The software allows the parts to be optimised by way of mould flow and stress analysis. Prototype moulds are sometimes built before the design is committed to mass production. The ABS plastic is heated to 232 °C (450 °F) until it reaches a dough-like consistency. It is then injected into the moulds using forces of between 25 and 150 tonnes and takes approximately 15 seconds to cool. The moulds are permitted a tolerance of up to twenty micrometres to ensure the bricks remain connected. Human inspectors check the output of the moulds to eliminate significant variations in colour or thickness. According to the Lego Group, about eighteen bricks out of every million fail to meet the standard required.