A lexical set is a group of words that all share a particular phonological feature. A phoneme is a basic unit of sound in a language that can distinguish one word from another. Most commonly, following the work of phonetician John C. Wells, a lexical set is a class of words in a language that all share a certain vowel phoneme. As Wells himself says, lexical sets "enable one to refer concisely to large groups of words which tend to share the same vowel, and to the vowel which they share". For instance, the pronunciation of the vowel in cup, luck, sun, blood, glove, and tough may vary in different English dialects, but is usually consistent within each dialect, and so this category of words forms a lexical set: what Wells, for ease, calls the set. Meanwhile, words like bid, cliff, limb, miss, etc. form a separate lexical set: Wells's set. Originally, Wells developed 24 such labels—keywords—for the vowel lexical sets of English, which have been sometimes modified and expanded by himself or other scholars for various reasons. Lexical sets have also been used to describe the vowels of other languages, such as French, Irish and Scots. There are several reasons why lexical sets are useful. Scholars of phonetics often use abstract symbols (most universally today, those of the International Phonetic Alphabet) to transcribe phonemes, though they may follow different transcribing conventions or rely on implicit assumptions in their exact choice of symbols. One convenience of lexical sets is their tendency to avoid these conventions or assumptions. Instead, Wells explains, they "make use of keywords intended to be unmistakable no matter what accent one says them in". This makes them useful for examining phonemes within an accent, comparing and contrasting different accents, and capturing how phonemes may be differently distributed based on accent. A further benefit is that people with no background in phonetics can identify a phoneme not by learned symbols or technical jargon but by its simple keyword (like or in the above examples). Standard lexical sets for English The standard lexical sets for English introduced by John C. Wells in his 1982 Accents of English are in wide usage. Wells defined each lexical set on the basis of the pronunciation of words in two reference accents, which he calls RP and GenAm. "RP" refers to Received Pronunciation, the traditionally prestigious accent in England. "GenAm" refers to an