Sit at a desk etc, not on a desk etc

Sit at a desk etc, not on a desk etc

Don’t Say:
The bank manager was sitting on his desk.

Say:
The bank manager was sitting at his desk.

Note:
Also sit at a table. But on a chair, on a bench, on a sofa, etc; in an arm-chair, in a tree or up a tree. A bird sometimes perches (= srts) on a tree