Triangles v fronds

Q: When is a triangle not a triangle?

A: When it's a woman's thigh.

The basis for this riddle comes towards the end of Tate Modern's current headline show in London, Henri Matisse: The Cut-Outs, and it got me thinking. Here's the background: aged 71, Matisse became confined to a wheelchair following surgery, and could no longer paint at an easel. Rather than dam his creative flow, he diverted it, using scissors, coloured paper and assistants to assemble colourful configurations of shapes, deceptive in their simplicity and joyous in their exuberance.

Although this technique occupied Matisse for the last decade...