Somewhere along the way, a very small group of English words, through dialectical divergence, acquired spelling and punctuation variants in the form of an odd appendage: the letters -st. Three of these terms are acceptable (but declining in use) in British English but deemed nonstandard in American English, while two others, strangely, have prevailed over earlier forms.
The phonological term for this type of change is excrescence, which although it simply means “outgrowth” is a word with unpleasant associations that should help writers (and speakers) of American English to remember to think twice before using one of the following three words:
1. Amidst: The preposition amid, meaning “among” or “during,” or “with the accompaniment of,” is often written (but rarely said as) amidst, even in American English, but it is considered colloquial and unsuitable for formal writing.
2. Amongst: The excrescent form of the preposition among, in some senses synonymous with amid(st), is perhaps even more frequently employed in informal American English writing (and speaking). However, amongst, like amidst, should be avoided in formal writing.
3. Whilst: Alone among these three words, the conjunction whilst is rarely used in American English, perhaps because it sounds especially affected; many users of British English also favor while. Its relative unpopularity, however, is counterintuitive in that it is relatively easy to pronounce, while amidst and amongst involve some mandibular gymnastics.
Though they have the same ending as amidst, amongst, and whilst, these words ending in the excrescent -st are standard:
4. Against: Anomalously, though against followed a path similar to those of the three words listed above, forming from the alteration of again to againes and then to againest before settling into its current form, the nonstandard variant prevailed. Again, as a preposition, has been relegated to dialect used for comic effect; indeed, in this context, it is often spelled agin to emphasize the drawled pronunciation, as in “I ain’t sayin’ nothin’ agin ya” (translation: “I’m not saying anything against you”).
5. Midst: This variant of the noun middle (from the Middle English term middest, an alteration of middes, which in turn is short for amiddes, meaning “amid”) is correct, though its survival is curious, considering that middle is easier to pronounce. The truncated form mid is acceptable only as a prefix in a hyphenated (mid-Atlantic) or closed (midafternoon) compound.