Once we get to twenty a logical sequence steps in where the sigle digits (1 - 9) are repeated in each decade up to 100. Counting from 20 - 100 all seemed quite logical to me until yesterday when I suddenly realised we have been creating yet another dilemma for young children learning to count. One of the common errors young children make when they first learn to count in the decades is to say, for example, "twenty-seven, twenty-eight, twenty-nine, twenty-ten (instead of thirty). In the past this has easily been corrected as it did not make any sense to say "twenty-ten". The problem is that the terms twenty-ten, twenty-eleven, and twenty-twelve have become legitimate terms of usage when refering to the date of the year. These terms seem to have replaced the more mathematically correct, for example, "two thousand twelve" (with or without the "and').
This is similar to the double use of the word "third", for example, as an ordinal number as in "I am third in line" and as a fractional number such as "I have a third of a cup of orange juice".