Two numbers that break the pattern
From thirteen to nineteen, English builds its numbers in a way that is easy to see: a unit plus “-teen”, meaning ten. Thirteen is three and ten, fourteen is four and ten, and so on. Then eleven and twelve do not fit. Nothing about the sound of “eleven” suggests “one” or “ten”, and “twelve” has little in common with “two”. Why are they different?
The answer is that eleven and twelve were never built on “-teen”. They come from an older and different way of counting past ten.
The Old English forms
In Old English, eleven was “endleofan” and twelve was “twelf”. Both are compounds, and their elements can be recognised by comparing them with related languages. The first part of “endleofan” is a form of “one” (compare the old Germanic roots seen in German “ein”), and “twelf” begins with a form of “two”. The second part of both words is generally explained as related to a word meaning “left” or “remaining”, which is connected to the verb “leave”.
On that reading, “endleofan” means something like “one left”, and “twelf” something like “two left”. The sense is that after counting ten, you have one remaining or two remaining. It is a form of counting past ten that does not name ten at all. This is the standard scholarly explanation, though the exact reconstruction of the older Germanic forms involves some uncertainty.
Evidence from relatives
The explanation is persuasive because it is not limited to English. Gothic, an early Germanic language recorded in the fourth century, had words for eleven and twelve that fit the same pattern, “ainlif” and “twalif”. German “elf” and “zwolf”, Dutch “elf” and “twaalf”, and Scandinavian “elleve” and “tolv” are all descendants of the same type of compound. The same formation appears outside the Germanic family: Lithuanian has “vienuolika” for eleven and “dvylika” for twelve, in which the final element is generally related to the same idea of “left”. Finding the same method in two branches of the Indo-European family is one reason scholars take the interpretation seriously.
Note that the ancestor of the “-teen” ending is a separate word. It descends from the word for ten, and in Old English the form was something like “-tiene”. It appears in thirteen to nineteen, where a unit is joined to it directly.
How the teens developed
The pattern for thirteen to nineteen is that the unit comes first, then ten, as in Old English “feowertiene” for fourteen. Over time the spelling and pronunciation wore down, and “ten” became “-teen” with a long vowel. Some units lost part of their form: the “thir-” of thirteen and the “fif-” of fifteen are shortened versions of “three” and “five”, related to the same sound changes behind “third” and “fifth”. The same shortening shows up in the ordinal numbers third and fifth.
Why did the language switch method after twelve? A simple answer is that the “leftover” pattern may have been enough for the first two numbers beyond ten, and that the more transparent “unit plus ten” pattern was then used for the rest. There is also a suggestion that the strong role of twelve in counting, seen in the dozen, may have kept twelve distinct. These are plausible ideas but not established explanations, and linguists treat the question of why the break happens at twelve as open.
A trace of an older arrangement
Some writers have suggested that the pattern shows an older way of counting in Germanic, in which people counted to ten, and then counted what was left. That idea is natural, but it should be held loosely. We know the forms, and we can interpret them as “left over”, but we cannot observe how people actually counted when these words were formed.
Related to this is the question of twelve itself. The importance of twelve in measurement, such as the dozen, the gross and twelve inches in a foot, belongs to the tradition of duodecimal counting and the traditional counting units. Whether the special status of twelve influenced the language is uncertain. It is equally likely that the counting words and the practical units developed on their own and reinforced each other.
Tens and the order of units
The structure of the tens also has history. Twenty derives from an Old English form meaning “two tens”. The older English way of saying numbers like 21 was “one and twenty”, with the unit first, which survives in the nursery rhyme “four and twenty blackbirds” and in German, which still says “einundzwanzig”. Modern English reversed this order, saying “twenty-one”, and the change shows that even regular parts of the system have moved over time. In the older pattern, as in “four and twenty”, the unit and the ten were linked by “and”.
An old and surprising use of “hundred” also appears in some medieval Germanic usage, where it could mean a “long hundred” of 120. That use is reported in Old Norse and in some English texts, and the details differ from source to source, so it is safest to say that “hundred” did not always name exactly 100.
Comparison with French and Romance
The Romance languages used a different compound. Latin had “undecim” (“one-ten”) and “duodecim” (“two-ten”), which became French “onze” and “douze”, Spanish “once” and “doce” and Italian “undici” and “dodici”. These also have forms of their own up to about sixteen, such as French “treize” and “seize”. So the Germanic idea was “left over after ten”, while the Latin idea was “one and ten”. French has another set of irregularities higher up, which are part of what makes French number names distinctive.
Summary
Eleven and twelve are irregular because they come from a different formula than the other teens. Old English “endleofan” and “twelf” are best understood as “one left” and “two left”, a pattern also found in Gothic and Lithuanian. The remaining teens, from thirteen to nineteen, use the transparent pattern of unit plus ten. The break at twelve may reflect the importance of that number, but that part is a guess. What is firmly known is that the words are compounds, not arbitrary.