- 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’.
Modern English usage. 2014.