A consonant is a speech sound that is articulated with complete or partial closure of the vocal tract. Consonants are letters that represent certain speech sounds, specifically sounds that involve blocking the air before it leaves the mouth, such as with the tongue, lips, or throat. Most letters of the English alphabet are consonants, except for a, e, i, o, and u, which are vowels. Consonants play a significant role in both spelling and pronunciation, as well as writing where sound is important, like poetry or music lyrics. The English alphabet has fewer consonant letters than the English language has consonant sounds, so digraphs like ⟨ch⟩, ⟨sh⟩, ⟨th⟩, and ⟨ng⟩ are used to extend the alphabet, though some letters and digraphs represent more than one consonant. The word consonant comes from Latin oblique stem cōnsonant-, from cōnsonāns sounding-together, a calque of Greek σύμφωνον sýmphōnon (plural sýmphōna, σύμφωνα) .