Ways it can matter:

• Vocabulary and Notation: Pd not Rd, sometimes Sd, with statics and centers of motion

• Basic Structure and Vocabulary: projective notation Plucker, Grassman, Cayley; body-hinge.

• Techniques to test: e.g. Coning, Projection, pure conditions as projective constructions;

• Change of Metrics: plane, sphere, hyperbolic

• Questions to ask: What happens with points at in_nity? - sliders; polarity and sheet structures.

• Recognizing Examples: Molecular frameworks; proteins; buildings;

• Analogies with other projective constraint systems: parallel drawing and scene analysis; multivariate splines.

Walter Whiteley

York University, Canada