A rectangle with sides u and v, drawn to scale, grows when x is nudged by a small step: a strip along the right edge of width delta u, a strip along the top of height delta v, and a small corner square where the strips meet. A slider shrinks the step. A ledger lists the two strips, the corner, the total growth, the corner's percentage share of the growth, and growth divided by the step. In square mode u and v are both x equal to three, so the strips give slope six; in rectangle mode u is three growing at rate one and v is two growing at rate two, so the strips give slope eight. The corner's share falls toward zero as the step shrinks.

Two strips and a corner

Nudge x; both sides of the rectangle grow. The new area is two strips plus a corner — and the corner has two small factors.