A letter is any character that isn't a number or a symbol. Lowercase letters in the English language and letter keys include a, b, c, d, e, f, g, h, I j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, and z. Uppercase letters are simply all the lowercase letters with the exception of 'i' and 'j'. Letters are used to print words on a page or screen. They can be displayed as numbers by using the ASCII table. Computer scientists have more than names for the letters of the alphabet; they also classify them. The eight letters that make up the English word "alphabet" are called "vowels". Vowels do not count as consonants because they do not produce sound when they are by themselves. Rather, they are responsible for making certain other letters sound like vowels. For example, if you say the letter "b" without making a noise, it makes the following letter "a" sound like a vowel. This is why groups of two or more consecutive letters can only contain letters that make sounds when they are by themselves. There are five categories of letters: consonant, semivowel, full vowel, half vowel, and mute (also called null). Consonants are letters that produce a sound when they are by themselves but not when combined with another letter.