A question about numbers.
Why are 16 - 19 and 21-29 one word (i.e. Diecisiete) but 31 - 99 are three words (i.e. treinta y uno?
2 Answers
This is a tricky one. My personal hypothesis is that words, grammar rules, and structures that get used most frequently are those most likely to hold onto and maintain older and more "irregular" forms, whereas words and phrases that are rarely or infrequently used find a way of settling into simple, easy to manipulate patterns. For example the verbs for "to be" which can't be avoided in daily conversation are some of the most irregular verbs in many languages (not just Spanish but also English and French). I think something like this may also explain the one-word form of 16-29. These are certainly smaller numbers and more likely to be used on a regular basis, especially when you stop to think of former, agrarian societies where people lived fewer years and were surrounded by smaller populations. This is just my unprofessional hypothesis.
Thank you for the question. I like to have a little linguistic philosophy on the Q&A forum. 
Los números cardinales en español se dicen en una sola palabra del 1 al 30. Es una regla gramatical.
