Some compound words are separated by a space (e.g. ice cream). Others are simply joined together (e.g. football, doorknob). Others still are hyphenated (e.g. long-term, off-topic).
Why is the handling of compound words not entirely consistent? What determines whether a new word will be spaced, joined, or hyphenated?
Consistency of the form is not at stake: you might however speak of lack of consistency in the spelling of compound words. The reason why there is not a uniform way of spelling them is that there were no directives as they started being used in hyphenated form or closed form, so people coining or using them might use one of the three forms rather than another.
Hyphenation as a compounding technique in English Nonetheless, hyphenated compounds have largely been ignored in the academic literature. Likely because of the common belief that using hyphenation to link words (such as punctuation practices) tends to be conventional (i.e., just a matter of spelling), only a few studies have explored hyphenated compounds, such as Mondorf (2009) and Kuperman and Bertram (2013).
As time went by, there has been much evolution and finally some compound words can be found for which the three forms can be considered correct.
Notice the great number of cases found for the apparently rare icecream, which is in fact only relatively rare.
The contemporary trend in spelling compound words would be to use hyphens more, but this question is quite complex as hyphens also tend to disappear (economy of writing, such as promoted by typing, for instance); see the article shown above (Hyphenation as a compounding technique in English) if curious enough about this evolution in English spelling: although not easy to follow this masterly study still provides much insight accessible to the layperson.
There is also a difference according to what variety of the language you consider.
(CoGEL III 4, Quirk et al., 1985) AmE inclines to fewer hyphens than BrE, preferring words to be written either open (separated by a space) several exceptions:either OPEN by a space) or SOLID (without separation) rather than hyphenated.
