Why doesn't "its" have an apostrophe?

I was told that the possessive apostrophe was originally a contraction.

"The bear's coat" was originally "the bear his coat" and the apostrophe appeared as the his was gradually shortened — so it would make sense that it is had an apostrophe but its didn't.

Here are some possessive pronouns:

• My neighbour
• Your friend
• His wife
• Her dog
• Its tail

None of these have apostrophes. See Martin Beckett's answer for the rest.

Professor David Crystal explains it in his book The Fight for English: How language pundits ate, shot, and left (Crystal 2006), pp. 134-135:

Its is just as possessive as cat's, but it doesn't have an apostrophe. Why not? Because the printers and grammarians [of the nineteenth century - Alex B.] never thought the matter through [emphasis mine - Alex B.]. They applied their rule to nouns and forgot about pronouns, thus creating an exception (along with the food is hers, ours, yours, theirs) without realizing it. And even if they had noticed, they wouldn't have done anything about it, for it's was already taken, as it were, as the abbreviation of it is.

I highly recommend this book to anyone interested in languages. 

Charles Fries (Fries 1927) speculates that it could also be so because "their plural forms do not end in s" (cf. one - one's - ones or other - other's - others, ft. 7)

For an excellent summary of how the rules concerning apostrophe use developed, see

Sklar, E. (1976). The Possessive Apostrophe: The Development and Decline of a Crooked Mark. College English, 38(2), 175-183. doi:10.2307/376342

If you want to learn more about how the grammarians of the past arbitrarily imposed their confusing rules - and didn't stick to them - see pages 197-198 in Doctrine of correctness in English usage, 1700-1800 by S.A. Leonard (Leonard 1962); for instance, Joseph Priestly argued in The rudiments of English grammar, which was published in 1772, the following (pp. 86-87):

On the other hand, in the same book, on page 11, he lists all the possessive pronouns without an apostrophe and he treats its separately, as the genitive form:

Thirteen years later, J. Mennye in An English grammar ; being a compilation from the works of such grammarians as have acquired the approbation of the public [...] argued diametrically the opposite of the convention earlier proposed by Joseph Priestly.

But in 1823, T.O. Churchill says the following in A New Grammar of the English Language:

I read through this thread and found that you really do not have an answer. The etymology of all these words, adjectives and pronouns alike, is confusing, to say the least. Additionally, it seems that there are conflicting reasons for everything. Some say that there used to be apostrophes with certain adjectives/pronouns, and some say they never existed. But let me try to help to make this simple. First and foremost, we need to recognize the different categories we have here. We have possessive adjectives and possessive pronouns. For example, hers is a possessive pronoun, while *her" is a possessive adjective, in our current context at least. Its can technically fall within each, even though we most commonly hear/see it as being the adjective. Moving on--

So, lets look at the personal pronoun we. In no way, shape, or form would we say we's, and if you know anything about etymology, you know that "we's" or "we" does not and cannot turn into "our" or "ours". This proves that our, along with other possessive adjectives are their own words. Now, let's move to those, for a second, to further my point. It would be easy to just say "the apostrophe was omitted" in cases like "our" to "ours" and so forth, but what about the other cases in which the words are more etymologically different; "mine" to "my", "who's" to "whose"? Exactly, they are just different words. Throughout the English language, we have many words that sound and look the same, and they might even be derivative of eachother. However, in English's current state, they are their own entity-- they are their own word, a different word. If you take a step back and think about things loosely, you will see that the same applies to "its". While it does seem weird when you try to think of it as you do, "its" really is just its own word.

That's my take. Take it or leave it.