Buzz
English number words make sense up to a point. From twenty-one to ninety-nine, the pattern holds: you say the tens place followed by the ones. But the teens break this rule. Not only does the 'ten' (from which 'teen' is derived) appear after the ones (so 10+7 is 'seventeen' and not 'teen-seven'), but 'eleven' and 'twelve' don't follow this pattern either.
Eleven and twelve come from Old English words endleofan and twelf, which trace even further back to an era when they were ain+lif and twa+lif. So, what does the –lif part mean? The most likely explanation from etymologists is that it comes from a root meaning 'to leave.' Ainlif means 'one left (after ten),' and twalif means 'two left (after ten).'
So why don't we have words like threelif, fourlif, fiflif, or sixlif? The answer lies in the historical evolution of number systems. Long ago, when these number words were first forming, most people had little need to differentiate numbers above ten. In fact, some ancient languages only had words for one, two, and 'many.' The base numbers up to ten were established first, and then the –lif suffix was added.
Perhaps there was once a threelif, fourlif type system, but 11 and 12 became more common in everyday usage. Many cultures based their number systems on 12, as it's divisible by the most numbers. You can count to 12 on one hand by using your thumb to count the three knuckles on each finger. (This is why we have the word dozen.) When 11 and 12 were used more often, their forms became fixed, even as other systems began to emerge.
This idea can be extended to other number words. The irregularities in pronunciation of numbers like twenty, thirty, and fifty (instead of twoty, threety, fivety) exist because we’ve been using these numbers for far longer than two hundred, three hundred, or five hundred. The word thousand is ancient, but originally it meant 'a great multitude,' which wasn't specifically numeric, but very useful. The numbers we used most often and earliest tend to be the ones that evolved the most irregularly.
The simple answer is that we created words for 11 and 12 long ago, calling them 'one left after ten' and 'two left after ten.' Since these numbers were more commonly used than higher ones, they became habitual and stuck in the language.
Kids are often the first to notice the oddities in language. A big thank you to five-year-old Katie English for this great question!
See Also...
Why does 'will not' become 'won't'? *Why isn't 'Arkansas' pronounced like 'Kansas'? *Why is there an 'R' in Mrs.?
