In articulatory phonetics, a consonant is a speech sound that is articulated with complete or partial closure of the vocal tract. Examples are [p], pronounced with the lips; [t], pronounced with the front of the tongue; [k], pronounced with the back of the tongue; [h], pronounced in the throat; [f] and [s], pronounced by forcing air through a narrow channel (fricatives); and [m] and [n], which have air flowing through the nose (nasals). Contrasting with consonants are vowels.Since the number of possible sounds in all of the world's languages is much greater than the number of letters in any one alphabet, linguists have devised systems such as the International Phonetic Alphabet (IPA) to assign a unique and unambiguous symbol to each attested consonant. In fact, the English alphabet has fewer consonant letters than English has consonant sounds, so digraphs like "ch", "sh", "th", and "zh" are used to extend the alphabet, and some letters and digraphs represent more than one consonant. For example, the sound spelled "th" in "this" is a different consonant than the "th" sound in "thin". (In the IPA they are transcribed [ð] and [θ], respectively.)