Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
free (adv., adj.), freely (adv.)
The adjective (a free spirit; no free lunch) poses no special problems, but the two adverbs, free and freely, are not interchangeable. All guests will be admitted free means “the guests won’t have to pay,” but All guests will be admitted freely means “they’ll be admitted without any restrictions as to numbers, time, or anything else.” To let the prisoner go free is “to set him permanently at liberty”; to let him go freely is “to let him come and go as he pleases, even though he is still technically a prisoner,” or it is “to let him go without any argument on his captors’ part, with no disagreement or reservation.” See FOR FREE.
