Each other and One another

Each other = ซึ่งกันและกัน
One another = ซึ่งกันและกัน (มากกว่าสอง)

Peter and Mary helped each other.
     = Peter helped Mary and Mary helped Peter.
We sent one another Christmas cards.
     = We sent them a Christmas card and they sent us a Christmas card.
They didn't look at each other.
     = X didn't look at Y and Y didn't look at X.