I or i is the ninth letter of the Latin Alphabet. (The English Language, and many other languages, usually uses the Latin Alphabet, when being written.)

The letter I is usually pronounced: Aee.

The letter before I is: H. The letter after I is: J.

Beyond this, there are a number of other uses of the letter I and i. Such as in....

There is also an interesting history of the letter I with the letter J.


I (always written in its Upper Case form), is a first-person singular personal pronoun, in the English Language.

For example, you could say....

I walked to the store, and bought some apples.

This is similar to the word Man in Persian.


Note that the letter I and the letter J used to be the exact same letter! And used to be used interchangeably.

This combined I / J letter made both the consonant sound we associate with J and the vowel sounds we associate with I, depending on the context.

We see this in names like: Januarius and Ianuarius. And: Janus and Ianus.

Roman Numeral

I or i is also the Roman Numeral for One (1).

There is no Roman Numeral before I. The Roman Numeral after I is: II.


In Chemistry, I is the Chemical Symbol for Iodine.


In Physics, I and i have a couple of uses, in addition to the uses that Mathematics uses for I and i... which often get pulled into Physics.

Electric Current

In Physics, I is used as a short form for Electrical Current.

For example, in Ohm\'s Law, we have I being used as....

I = VR

Or in other words, the Electrical Current equals the Voltage divides by the Electrical Resistance.

Moment of Intertia



In Mathematics, i has a number of uses.

Complex Numbers

In Mathematics, i (always written in its Lower Case form) is used in Imaginary Numbers and Complex Numbers. With this, i and -i are used to represent the two sqaure roots of negative one. Or in other words, we defined i as....

i2 = -1

And thus we have....

 i = √-1
-i = √-1

i is called the Imaginary Unit.

Note though, when Complex Numbers are being used in Phyiscs were I is being used for Electrical Current, Physists instead used something called Operator J (for the Imaginary Unit instead of i).

This Operator J is simplier that it sounds. It really just replaces i with j in....

j2 = -1

And thus....

 j = √-1
-j = √-1

And thus uses j to represent the Imaginary Unit.

Note, this use of j should not be confused with the j in Quaternions.


Quaternions define i, j, and k as....

i2 = -1
j2 = -1
k2 = -1

i × j = k
j × k = i
k × i = j

j × i = -k
k × j = -i
i × k = -j

Note that unlike Multiplication with Real Numbers and Complex Numbers, Multiplication with Quaternions is not Commutative.

Mathematical Indexing

Also in Mathematics, i (almost always written in its Lower Case form), is used in indexing. For example....

Σ xi


For those involved in Web Development writing HTML, there is an HTML i Element, as in....

    I punched him, not once, not twice, but <i>thrice</i>!

This HTML Element is used to make text Italics. And would get rendered as....

I punched him, not once, not twice, but thrice!


For those involved in Programming (like Programmers, Developers, and Software Engineers), i is frequently used as a indexing variable, in a number of Programming Languages.

For example, C and languages with C-like syntax (like C++, Java, JavaScript, and PHP), often used a variable i like....

for (i=0; i<n; i++) {

    /* Stuff happens here */


I\'ve read that this is likely a carry over from the used of i for indexing in Mathematics.

However, I\'ve read that the first Programming Language to have favored i as an indexing variable may have been Fortran.

If Fortran you can declare your variable if you wanted to (and thus set the type of the variable). But if you don\'t then you get a kind of default declaration based on the first letter of the variable.

If the variable started with the letters i through n, then the default declaration would set it to an integer (which would be suitable for indexing). (Variables that started with any other letter were reals.)

-- Mirza Charles Iliya Krempeaux
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