(There is a sixth, which is the internet. This should have been up on Friday but I have been internet-deprived for six days and only managed to post it today by driving to Starbucks and paying for unnecessary food in order to use free wifi.)
Besides being extremely useful glue for sticking consonants together into words, AIEOU was used as a cryptic symbolic device by the Habsburg emperors. Frederick III introduced it in the fifteenth century and used it as a 'Frederick woz 'ere' signature on buildings and such like. Just before he died, he said it stood for Alles Erdreich ist Oesterreich untertan - All the world is subject to Austria.