Number 13 in an order — the one that comes after the twelfth. Written short as 13th. You use it in three ways. First, before a noun: 'my thirteenth birthday', 'the thirteenth floor', 'the thirteenth of May'. Second, on its own with 'the', when we both know what we are counting: 'She was the thirteenth to arrive.' Third — and students forget this one — it can mean one piece out of thirteen equal pieces: 'a thirteenth of the total'. Same word, quite a different job. In dates, British people write '13 May' and say 'the thirteenth of May'; Americans write 'May 13' and say 'May thirteenth'. Notice that we say 'the' in speech even when it is not written. One small thing worth knowing about life in English-speaking places: many people think 13 is unlucky, so some hotels and tall buildings have no thirteenth floor — the lift goes 12, 14. This is much like some buildings here skipping the fourth floor.