A consonant cluster is a group of consonants that are positioned together without any intervening vowel. In English, for example, the groups /spl/ and /ts/ are consonant clusters in the word "splits". Consonant clusters can appear at the start or end of a word, such as "st" in "stay," "sk" in "task," "nt" in "went," and "nd" in "sound". The longest possible cluster in English is three consonant sounds at the start, such as "splash," and four at the end, as in "twelfths". Consonant clusters can be challenging for non-native speakers of English because not all languages allow so many consonants together without intervening vowel sounds. It is important to distinguish consonant clusters from consonant digraphs, where two letters make one sound. Teachers may focus on one consonant cluster for a set period of time, for example, they may choose to concentrate on "mp" for a week, giving children phonics activities involving reading and writing words containing "mp," such as "jump," "bump," "mumps," and "lamp".