Etymology and pronunciation of arch-, archi-

The prefix arch-, archi- “chief, principal; extreme, ultra; early, primitive,” derives from Latinized Greek arkh-, arkhi-, the combining form of arkhos “chief.” Usually, arch- is pronounced like “arch” (ɑrtʃ), and archi- sounds like “ark” (ɑrkɪ), although archangel (ɑrkeɪndʒəl) is a notable exception.

Is there an etymological or other reason for pronouncing the two prefixes differently? Did they perhaps enter English at different times? Likewise, is there any pattern to which nouns use arch- (e.g., archenemy, archfiend) versus archi- (archiepiscopal, architect)? Why is archangel an exception to the pronunciation rule?

Answer

An Etymological Dictionary of the English Language (1888) by Walter W. Skeat answers all your questions at once.

ARCH-, chief; almost solely used as a prefix. (L.--Gk.) Shak. has 'my worthy arch and patron,' Lear, ii. I.61; but the word is harshly
used, and better kept as a mere prefix. In arch-bishop, we have a
word in very early use; A.S. erce-bisceop, arce-bisceop (Bosworth).
Thus arch- is to be rightly regarded as descended from A.S. arce-,
which was borrowed from Lat. archi- (in archi-episcopus), and this
again from Gk. άρχι in
άρχιεπίσκοπος,
an archbishop. Gk. άρχός to be first;
cf. Gk. άρχή, beginning. Cf. Skt. arh, to be

worthy; Curtius, i.233. The form of the prefix being once fixed, it
was used for other words. Der. arch-bishop, arch-deacon, arch-duke,
arch-duchy, &c. In the word arch-angel, the prefix is taken
directly from the Greek; see Archi-.
ARCHI-, chief; used as a prefix. (L.--Gk.) The older form is arch-, which (as explained under Arch-) was a modification of A.S. arce-, from Lat. archi-. The form archi- is of later use,
but borrowed from the Lat. directly. Gk.
άρχί-, prefix. See Arch-. Der. archi-episcopal, archi-episcopy, archi-diaconal. In the word arch-angel, the final i of the prefix is dropped before the vowel following. In the word arche-type, the prefix takes the form
arche-.

Another edition of this dictionary (revised in 1910; reprinted in 2005) adds a couple of comments to the explanation.

ARCH- <...> In the word arch-angel, the ch remained hard (as k) in the Romance languages, on account of the a following; cf. Ital. archangelo, Span. arcangel.
ARCHI- <...> The same prefix also forms part of the words archi-mandrite, archi-pelago, archi-tect, archi-trave.