Why do people get rolls on the back of their neck?

[99bottles]

Why do people get rolls on the back of their neck?

I found the sentence above HERE.

I wonder, shouldn't it be backs instead of back, since people and necks are plural? I googled it and found examples with back of their necks as well as ones with backs of their necks. I'm confused.

 

[5jj]

I wonder, shouldn't it be backs instead of back, since people and necks are plural?

You have used singular 'neck' in your example and plural 'necks' in your question. Which did you mean?

 

[99bottles]

You have used singular 'neck' in your example and plural 'necks' in your question. Which did you mean?

Oh, sorry, I thought it was back of their necks.