A 'bob' was the slang word for a Shilling, which was worth 12 old pennies. Following decimilisation in 1971, a Shilling was worth 5 new pence. The old 'ten bob note' (10 shillings) was the equivalent of 5 Florins, or 4 Half Crowns, or 2 Crowns. After decimilisation, it was worth 50p.