Which is correct, "cill" or "sill"?

When I was an architectural technician, I used the spelling cill (e.g. window cill). I knew of one architect who used sill and stated that this was the older and more correct form.
My Concise Oxford Dictionary gives both but is weighted more towards sill.
Which version is the more accepted?

Solution 1:

In greater use? Sill.

Middle English sille, from Old English syll; akin to Old High German swelli beam, threshold First Known Use: before 12th century

This Google Ngram shows sill as being in greater use than cill.

Solution 2:

Which is correct?

Both. They're both attested and have been used in perfectly good English.

Which should you use? Almost certainly sill because it's better-known, while cill is rare enough that some may not recognise it, or think it incorrect even though it isn't.

Solution 3:

The full Oxford English Dictionary includes "cill" only as a historical variant spelling in the entry for "sill". Specifically, the entry's variant forms section includes "18 cill", meaning that they're only aware of it being used in the 1800s (as distinct from "18- cill", meaning 1800s to present).

As such, I would recommend using "sill"; both are correct but "cill" is seldom used in current English – add me to the list of native speakers who can't recall having seen it before.

Solution 4:

When I was at technical college in England in the 1970s, the ex-tradesmen/lecturers used the spelling cill which seems to tie in with the references here (window cill, cill of lock gate) to the use of cill as a technical term among professionals like builders, architects or engineers and the sill form among the general population.

This also explains the wide difference in frequency; there are fewer professionals than the general population. Hence the mere frequency of a word is not supportive of any arguments about correctness; however, the argument about wanting to APPEAR correct should be modified to match the audience for which one is writing; professional or general.

Solution 5:

While at college in the 1970's we were taught cill by our lecturer and that is how I have always spelt it for windows etc. I have taught in a college for 18 years and that is the spelling I teach.