H.W. Fowler, A Dictionary of Modern English Usage: a-, an-:
a-, an-, not or without. Punctilious word-making requires that these should be prefixed only to Greek stems; of such compounds there are some hundreds, whereas Latin-stemmed words having any currency even in scientific use do not perhaps exceed half a dozen. There are the botanical ascapular and acaulous, the biological asexual and acaudate, and the literary amoral. This last being literary, there is the less excuse for its having been preferred to the more orthodox non-moral. Amoral is a novelty whose progress has been rapid. In 1888, the OED called it a nonce-word, but in 1933 full recognition had to be conceded. These words should not be treated as precedents for future word-making.
