Two ice-cream carts on the same beach always end up side by side

Harold Hotelling showed in 1929 that rivals fighting for market share tend to minimise the differences between them rather than maximise them. His model of two vendors on a straight beach predicts both drift to the midpoint, even though spreading toward the ends would serve customers better. The same pull explains why competing petrol stations and political parties often converge.

— Harold Hotelling, Stability in Competition — The Economic Journal, Vol. 39, No. 153, 1929