Why does the 'i' in 'explain' disappear when written as 'explanation'? [duplicate]

I often catch myself trying to write ?explaination, phonetically spelling the word in my head. To my chagrin I get part way through and have to stop myself.

So I’m wondering why is the i dropped? I can’t think of other words ending ‑ain that take the ‑ation suffix for comparison.

Solution 1:

Expanding on ΜετάEd’s answer, the alternation between long /e:/ and short /a/ (or reduced version thereof, schwa or a mid-high vowel) is now frequent in Modern English (and harks back to the Great Vowel Shift).

So, alongside expl[e:]n ~ expl[ə]nation, you also have:

• ex'pl[e:]n ~ expl[æ]natory
• ins[e:]ne ~ ins[æ]nity (insane ~ insanity)
• in[e:]ne ~ in[æ]nity (inane ~ inanity)
• n[e:]tion ~ n[æ]tionality (nation ~ nationality)
• [e:]ble ~ [ə]bility (able ~ ability)

As you hunched and ΜετάEd pointed out, there is an orthographic oddity to explain, in that its /e:/ sound alternates with a lax vowel, but it is, orthographically, ai. The nearest I can come to like examples is:

• retain ~ retention (this too is an etymological oddity, examples like contend ~ contention, intend ~ intention_, and so on, would lead one to expect retend ~ retention)
• inveigh ~ invective (where the origin of eigh is again etymologically odd, coming from invehere)

As the parenthetic comments make clear, these are as much oddities as the case you identified.

Solution 2:

The question should be: where did that i come from.

If we look at etymonline we find the following: (emphasis mine)

explain (v.)
early 15c., from Latin explanare "to make level, smooth out;" also "to explain, make clear"

Originally explane, spelling altered by influence of plain. Also see plane (v.2). In 17c., occasionally used more literally, of the unfolding of material things: Evelyn has buds that "explain into leaves" ["Sylva, or, A discourse of forest-trees, and the propagation of timber in His Majesties dominions," 1664]. Related: Explained; explaining; explains.

So it seems that the form explane, probably pronounced much like the current version, lost its final e and to reflect the pronunciation in the spelling, an i was added.

Solution 3:

OEtmD says explain was “originally explane, spelling altered by influence of plain”.¹

There have been many different ways that English words have been spelled to indicate pronunciation. The word plain, for example, has been spelled plain, plane, plaine, playn, playne, pleyn, plegn, and plen.²

Over time, spelling has become more consistent: for example, we use plain and plane, and have stopped using the others. But spelling has not been regularized to the point where every similar word uses the same spelling rule to indicate pronunciation.

There is not really a particular reason why we ended up with a particular spelling: or, rather, the reason is simply that the language evolved to be so.