YAT AND ETA
Today I realized that my interpretation of the early Slavic vowel yat as [ɛː] (< *ai) sounded like the classical value of the Greek letter Η eta. Since Cyrillic is an offshoot of the Greek alphabet, one might expect yat to have been written with an eta-based Cyrillic letter. But of course eta was actually the model for the Cyrillic letter И <I> because eta had raised to [i] by the 4th century AD, long before Cyrillic was created in the late 9th century. [ɛː] was long gone in Greek, so a non-Greek letter was created for yat: Ѣ.
Ѣ looks like a derivative of the front yer letter Ь [ɪ] which in turn looks like a derivative of the Glagolitic front yer letter Ⱐ. But it is strange that a lower mid long vowel was written with a modified lower high short vowel rather than, say, with an additional stroke (like Czech ě which is nowadays used to transliterate yat). I don't see any resemblance between Ѣ and its Glagolitic counterpart Ⱑ.
5.14.23:24: According to Wikipedia, Schenker (1995) thought Ⱑ might be from Greek alpha Α. That makes a lot of sense if yat were [æ].
Modern reflexes of yat vary considerably in height from [ja] with a
low vowel in eastern Bulgarian* to [i] in Ukrainian.
*Eastern Bulgarian has two reflexes of yat: [ja] and [ɛ]. The former is in stressed syllables not followed by front vowels. The latter occurs elsewhere.