Orthography: open, hyphenated, or solid (closed up)
English uses many open compound nouns, a large subclass of which, by convention in accepted English orthography, are not closed up (not solidified) and are sometimes optionally hyphenated in attributive position (that is, when functioning as a noun adjunct). Examples are high school, kidney disease, and file format. Although some other languages would close up these nouns’ components (for example, German usually does so), English has a tendency whereby it closes up only certain ones, usually only ones in which the head noun is monosyllabic (and even within that category, only sometimes, and in a way that is not fully standardized). For example, data set and dataset, or file name and filename, are accepted alternative forms, but file format, data format, and data analysis, which have multisyllabic heads (format and analysis), can only be spelled as open in accepted English orthography. This pattern holds for countless nouns with few exceptions. For the class with monosyllabic heads, there is a tendency that “compounds tend to solidify as they age,”[5]: 368 which is how a term such as data set becomes dataset, pin-up becomes pinup,[5]: 368 coal mine becomes coalmine, bottle cap becomes bottlecap, and so on. Such alternative forms usually continue to coexist in accepted use; style guides often convene on preferred dictionaries as a way of achieving consistency, by declaring that the headword form there will be the default styling for each such term.
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Orthography: open, hyphenated, or solid (closed up)
English uses many open compound nouns, a large subclass of which, by convention in accepted English orthography, are not closed up (not solidified) and are sometimes optionally hyphenated in attributive position (that is, when functioning as a noun adjunct). Examples are high school, kidney disease, and file format. Although some other languages would close up these nouns’ components (for example, German usually does so), English has a tendency whereby it closes up only certain ones, usually only ones in which the head noun is monosyllabic (and even within that category, only sometimes, and in a way that is not fully standardized). For example, data set and dataset, or file name and filename, are accepted alternative forms, but file format, data format, and data analysis, which have multisyllabic heads (format and analysis), can only be spelled as open in accepted English orthography. This pattern holds for countless nouns with few exceptions. For the class with monosyllabic heads, there is a tendency that “compounds tend to solidify as they age,”[5]: 368 which is how a term such as data set becomes dataset, pin-up becomes pinup,[5]: 368 coal mine becomes coalmine, bottle cap becomes bottlecap, and so on. Such alternative forms usually continue to coexist in accepted use; style guides often convene on preferred dictionaries as a way of achieving consistency, by declaring that the headword form there will be the default styling for each such term.