The diol formed by joining C-1 of one cyclopentane ring to C-1 of another cyclopentane ring, each of these two carbons also carrying one OH group (1,1'-bi(cyclopentyl)-1,1'-diol), is treated with acid. The main product is

Ionisation of either hydroxyl gives a tertiary cation on one ring carbon. A ring $\mathrm{C}-\mathrm{C}$ bond of the other carbinol carbon migrates to it, so the ring bearing the remaining OH expands from five to six members, while the carbon that received the migrating group becomes the spiro centre of the original five-membered ring. The carbonyl is next to the spiro atom, giving spiro[4.5]decan-6-one, $\mathrm{C}{}_{10}\mathrm{H}{}_{16}\mathrm{O}$.