yer vs back yer

yer and back yer both are nouns.

Word	Noun	Adjective	Verb	Adverb
yer	Yes	No	No	No
back yer	Yes	No	No	No

As nouns, back yer is a hyponym of yer; that is, back yer is a word with a more specific, narrower meaning than yer:

yer: Either of two letters in Cyrillic alphabets, which originally represented phonemically the ultra-short vowels in Slavic languages.
back yer: The Cyrillic letter Ъ/ъ, which in modern languages serves to denote a hard (non-palatalized) consonant.

Other hyponyms of yer include hard sign, front yer, soft sign, strong yer, weak yer.

yer (noun)	back yer (noun)
Either of two letters in Cyrillic alphabets, which originally represented phonemically the ultra-short vowels in Slavic languages.	The Cyrillic letter Ъ/ъ, which in modern languages serves to denote a hard (non-palatalized) consonant.