The prefix en- (from French) has a variant spelling em-. (This is also associated, although I believe imperfectly, with the use of the sound /m/ in the pronunciation of the prefix.) Although the general pattern of its use seems fairly clear (it shows up before labial consonants such as P and B), I've been finding it hard to give an exact description of its occurrence.
The Oxford English Dictionary says
In modern orthography and pronunciation en- becomes em- before b and p, and occasionally before m.
This rule was not fully established in spelling before the 17th cent.; in Middle English, as in Old French and Old Spanish, enb-, enp- are more frequent than emb-, emp-, though the latter may perhaps represent what was the actual pronunciation.
One thing that I find interesting is that even though this prefix is related to and often has been interchanged with the Latin-derived in- prefix, the rules for which forms to use are not entirely the same for both of these prefixes. The in- prefix regularly assimilates in English to ir- before R (as in irradiate) and to il- before L (as in illuminate), while the en- prefix does not (as in enrich, enlarge). And the rules for these prefixes are different from the rule for the native English prefix un-, which is regularly invariant, being spelled with N regardless of the identity of the following sound or letter.
One thing that isn't covered in the OED entry is possible exceptions or "edge cases". Can anyone give a more detailed description of the distribution of the two forms of this prefix? I'm hoping to have a comprehensive description of the use of en- vs. em- in present-day English; any further information about the use of these two spellings in earlier stages of English, and how the spelling of the prefix developed/changed over time, would also be welcome.
For this question, I'm only interested in the verb prefix, not the etymologically distinct negative prefix that is found in a few words like enmity.