In Harry Potter III, my attention was caught by the following construction:
Hermione and Ron were looking daggers at each other, and when they got into class, they seated themselves either side of Harry;
Why the preposition "on", or maybe another one, was not put before "either side"?
At first, I was thinking that the answer may be connected with the transitivity of the verb "seat", which was used here instead of the intransitive "sit" / "sit down". But then I searched for another examples in HP. That's what I have found:
He and Hermione sat down on either side of Ron
They went to sit down either side of her.
the Bulgarian Beaters <..> had landed either side of Mostafa
<..> and settled themselves in chairs on either side of him.
I see virtually no difference between "on either side" and "either side". Am I right? Can I use them interchangeably in speech and writing?