English writes the pronoun "I" as a capital letter every time, but nobody knows for certain why. The capital "I" took hold in the Middle English period, and historians and linguists offer several explanations for it, from giving the word more weight to making a single letter easier to read in handwriting.
If you write correctly, you capitalize "I" every time you use it as the first-person pronoun. Yet this is unusual. Most languages write their word for "I" in lower case: German writes ich, French je, and Spanish yo. So why does English treat it differently?
Where the word I comes from
English has been around for many centuries, and it has changed a great deal along the way. Most of its grammar and spelling rules have a history that can be traced. The capital "I" is an exception: historians and linguists agree that they have not found the true explanation for it. What is known is that it developed during the Old and Middle English periods.
In Old English, the pronoun was ic, a relative of German ich. Over time it took many forms, and Middle English texts write it in several ways, such as ich, ic, or simply "i". By the end of the Middle English period, the short form had won: the final consonant was dropped, and only "I" was left. Even so, the reason why this one letter came to be written as a capital remains unclear.
For the other words that start with a capital letter in English, see the rules of capitalization.
Theories about the capital I
There are several explanations for why English ended up capitalizing "I". Two facts are worth knowing first. The other personal pronouns, such as we, you, and me, are not capitalized. And when the short form "i" first appeared, it was written in lower case.
It gave the word more weight
Some scholars believe that once the final consonant was dropped, a word of one small letter seemed to carry too little weight. Writing it as a capital gave it importance, especially in a statement about oneself.
It put the writer first
Another theory is psychological. By writing "I" large, writers stressed their own importance rather than that of the subject. This explanation is popular, but it is also the hardest one to prove.
It was easier to read
The last explanation looks at handwriting rather than at the language itself. A lowercase "i" standing alone looked weak and was easy to overlook. In handwritten manuscripts, it could also be smudged, or read as part of the word next to it. Historians believe that scribes wrote it taller so that it would not become illegible.
From Chaucer to text messages
Some historians point to the time of Geoffrey Chaucer, who wrote The Canterbury Tales in the late 1300s, when a taller "I" was used in writing. From then on, the capital seems to have become the norm.
In quick text messages and chats, some people type a lowercase "i", usually because they want to write faster. In any other kind of writing, the capital is required, and to most readers a lowercase "i" now looks odd and weak on the page.
A capital letter is one way to make a word stand out on the page. Italics are another: see the rules for italics.