1. A computer-implemented method of sketching a real-world object, comprising: providing a set of geometric templates stored in a computer memory, each geometric template in the set specifying a respective basic geometric shape; enabling end-user extension of the set of geometric templates, including enabling end-user definition and creation of an additional basic geometric shape; and in response to end-user application of the extended set of geometric templates, generating on a computer display a sketch of a real-world object to serve as a basis of a feature in a CAD model representing the real-world object.