Habla a sus espaldas: Some questions.
I'm aware the phrase "Habla a sus espaldas" means "He/she/it/you talk/s behind his/her/its/your back" but I have a couple of questions about it.
Firstly the SD dictionary offers both "la espalda" and "las espaldas" for singular "back." Which is more common and is there any reason for using one over the other? Also, is plural "backs" represented exactly the same way, so "sus espaldas" could additionally equal "their backs"?
Secondly, if I wanted to alter the sentence and say "He (etc) talks behind someone's back" I would have said "Habla a las espaldas de alguien" but apparently I would have been wrong, because the SD dictionary says that to do something behind someone's back is "Hacer algo a espaldas de alguien" without the definite article before "espaldas." I thought I was finally getting a feel for when and when not to use the definite article and the absence of one here feels wrong to me. Is this just because it is a phrase or am I missing something else?
2 Answers
Many of these expressions like "habla a espaldas" de alguien or "habla a mis/tus/sus espaldas" which is seen/heard at times, are idiomatic, and do not follow any logical rule.

sorry i dont know the phrase but alguien is singular so, la espalda de alguien. I would say you are corect with your translation of he / she speaks behind their back (but plural in spanish )