Lego pieces of all varieties comprise a universal system. Despite variation in the design and purpose of individual pieces over the years, each remains compatible in some way with existing pieces. Lego bricks from 1958 still interlock with those made in the current time, and Lego sets for young children are compatible with those made for teenagers.

     Each Lego piece must be manufactured to an exacting degree of precision. When two pieces are engaged they must fit firmly, yet be easily disassembled. The machines that make Lego bricks have tolerances as small as 10 micrometre.