However, by the time the two friends had reached the second landing on the back stairs a terrible commotion was under way in the little yard.

They rushed to an open window at the end of the hall and looked down.
A thin board fence separated the flat's back yard from that used by the branch post-office.

In the latter place lived a collie dog.

He and Alexander had smelt each other out, blowing through the cracks of the fence at each other.

Suddenly the quarrel had exploded on either side of the fence.