MONOCULAR O Ꙩꙩ, BINOCULAR O Ꙫꙫ, DOUBLE MONOCULAR Ꙭꙭ, and MULTIOCULAR O ꙮ are used in words which are based on the root for 'eye'. The first is used when the wordform is singular, as ꙩкꙩ; the second and third are used in the root for 'eye' when the wordform is dual, as Ꙫчи, ꙭчи; and the last in the epithet 'many-eyed' as in серафими многоꙮчитїи 'many-eyed seraphim'. It has no upper-case form.

Wikipedia:

It was documented by Yefim Karsky from a copy of Psalms from around 1429, now found in the collection of the Trinity Lavra of St. Sergius, and subsequently incorporated into Unicode as character U+A66E.

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