In an alphabetic writing system, a silent letter is a letter that, in a particular word, does not correspond to any sound in the word's pronunciation. In linguistics, a silent letter is often symbolised with a null sign . Null is an unpronounced or unwritten segment. The symbol resembles the Scandinavian letter Ø and other symbols. English One of the noted difficulties of English spelling is a high number of silent letters. Edward Carney distinguishes different kinds of "silent" letters, which present differing degrees of difficulty to readers. Auxiliary letters which, with another letter, constitute digraphs, i.e. two letters combined which represent a single phoneme. These may further be categorized as: "Exocentric" digraphs, where the sound of the digraph is different from that of either of its constituent letters. These are rarely considered "silent". Examples: Where the phoneme has no standard single-letter representation, as with consonants for as in sing, for as in thin or as in then, diphthongs in out or in point. These are the default spellings for the relevant sounds and present no special difficulty for readers or writers. Where standard single-letter representation uses another letter, as with in enough or in physical instead of . These may be considered irregular for writers, but less difficult for readers. "Endocentric" digraphs, where the sound of the digraph is the same as that of one of its constituent letters. These include: Most double consonants, as in clubbed; though not geminate consonants, as in misspell. Doubling due to suffixation or inflection is regular; otherwise, it may present difficulty to writers (e.g. accommodate is often misspelled), but not to readers. Many vowel digraphs, as , , in leave (cf. accede), achieve, eulogy (cf. utopia). The discontiguous digraphs, whose second element is "magic e", e.g. in rate (cf. rat), in fine (cf. fin). This is the regular way to represent "long" vowels in the last syllable of a morpheme. Others, such as (which is in effect the "doubled" form of ), as in guard, vogue; as in bread, heavy, etc.; , as in aerial, oedipal. These may be difficult for writers and sometimes also for readers. Dummy letters with no relation to neighboring letters and no correspondence in pronunciation: Some are inert letters, which are sounded in a cognate word: e.g. in damn (cf. damnation); in phlegm (cf. phlegmatic); in practically (cf. practical); in ballet (cf. balletic). If the cognate is obvious,