1. A computer-aided design apparatus ( 2 ) for the design of three-dimensional objects to be fabricated, the apparatus comprising: a base generator operable to generate a topology for a design of a three-dimensional object by generating a plurality of different generations of bases, wherein each generation of bases comprises a plurality of bases, each base comprising a local coordinate system, and wherein the bases in the second and any subsequent generation are located in the local coordinate system of a base in a preceding generation; a base identifier operable to identify bases having a defined relationship; and a content generator operable to generate content objects for the identified bases.