Choosing between the prefixes e- and ex-

Question

Before a consonant on can use either version of the prepositions e/ex. Both seem to appear in prefixes as well, but ex- is often assimilated. It seems that, for example, words beginning with F take ex-: efficere, efferre, effundere… On the other hand, words beginning with L take e-: eligere, elevare, eloqui… My examples are verbs, but the pattern does not seem to be restricted to them.

This little observation suggests that some consonants (like L) require the prefix e- while others (like F) require ex-. Which ones take e- and which ones ex-? Are there perhaps consonants that can take either one? Is there an easy way to memorize or understand the division to two consonant types?

Related question about the prepositions rather than the prefixes: Why “ex nihilo” instead of “e nihilo”? — Asteroides, Jan 23 '17 at 23:20
I suspect it has to do with whether the consonant following is voiced or unvoiced (we have enervo, emendo, egredior but exsorbeo, expendo, extero). — Joel Derfner, Feb 02 '17 at 18:02
@JoelDerfner Good theory! I'll go through the consonants and see if it's true. If you have any ideas as to why it should be that way, please write an answer. I have no clue, other than it sounds better that way. — Joonas Ilmavirta, Feb 02 '17 at 19:28

Joonas Ilmavirta · Accepted Answer · 2017-02-03T09:31:23.057

I went through all Latin consonants. All consonants seem to prefer only one of e- and ex-. I have only listed one example per consonant. There are no (or only few and rare) verbs starting with X or Z.

The prefix e-:

B: (e)bibere
D: (e)ducere
G¹: (e)gerere
I²: (e)iurare, eicere < iacere
L: (e)loqui
M³: (e)mergere
N: (e)numerare
R⁴: (e)rumpere
V⁵: (e)vertere

The prefix ex-:

C: (ex)cedere
F: (ef)ferre
P: (ex)pellere
Q: exquirere < quaerere
S⁶: (ex)secare
T: (ex)tollere

¹ L&S mentions exgurgitare as a synonym of egurgitare. I found no other such verbs.

² I only included the consonantal I (or J). For vocalic I the prefix is ex-: ire > exire.

³ L&S mentions exmovere as a synonym of emovere. Otherwise e- is always used before M.

⁴ L&S mentions exradicare as a synonym of eradicare. There is also a rare exrogare.

⁵ I only included the consonantal V. For vocalic U (or V) the prefix is ex-: urere > exurere.

⁶ The exs- can become ex-; consider existere < sistere.

Conclusion: Voiced consonants take the prefix e-, unvoiced ones take ex-. There are very few exceptions. I have no idea why this is the way it is, other than it sounds more natural this way. See TKR's comment below for an explanation.

This rule is part of a larger pattern: Latin generally disallows s before a voiced consonant, and historically such an s was lost, along with any consonant that immediately preceded it, and with compensatory lengthening of the preceding vowel. For example, vēlum 'sail' < wekslom, āla* 'wing' < aksla, trēdecim* '13' < *trēsdekim. It's thought that in such cases the [s] first became [z] by voicing assimilation and later was lost. — TKR, Feb 03 '17 at 00:57

Choosing between the prefixes e- and ex-

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