- by, by-, byeThese three forms have different functions. By is a preposition or adverb (Come by / By the river); by- is a prefix meaning ‘secondary, subordinate’ (forming words such as byroad and bypass. Hyphenation practice varies: byline (a line in a newspaper column giving the writer's name) and byname, and the two mentioned above, tend to be written as one word whereas by-law, and longer forms such as by-election and by-product, are hyphenated. In some cases, a variant spelling bye- is also found (e.g. bye-law), but this is best avoided; and bye is a noun meaning ‘something additional or left aside’ (e.g. in cricket and golf). The idiom by the by (in which the second by is a variant of bye) means ‘by the way, incidentally’.————————by prep.By has so many functions that care should be taken to avoid ambiguity of the kind typified by the sentence, more hilarious than truly ambiguous, He was knocked down by the town hall. It is better to use another preposition such as close to or in front of. Fowler warned against the use of too many bys (in different senses) in one sentence, as in (not his example): Send stories by reporters by fax by the end of Friday. Such sequences are more likely to occur in more hurried forms of spoken English.
Modern English usage. 2014.