Behind/in back of/at the back of

What's the difference in meaning between "behind" "in back of" and "at the back of"?
Thank you very much for your reply.

They are nearly synonymous in many situations, and yet they are not truly synonyms.

"in back of" is a sort of intermediate form which can be forced to serve as either "behind" or "at the back of".

Katherine stood at the back of the room.
?Katherine stood in back of the room.
*Katherine stood behind the room.
She stood behind the wall.
*She stood in back of the wall.
*She stood at the back of the wall.

"We sat behind the bus" has us outside of the bus.
"We sat at the back of the bus" has us inside the bus.
"We sat in back of the bus" can be interpreted in either way.

"We drove behind the bus" is quite different from "We sat at the back of the bus".
"We drove at the back of the bus" is a bit nonsensical.

"Secret plans were being made behind his back."
*"Secret plans were being made in back of him."
*"Secret plans were being made at the back of his back."
*"Secret plans were being made at the back of him."

Who's that behind me?
?Who's that in back of me?
*Who's that at the back of me?

"Suddenly, there was a loud crash at the back of the stage" is not the same as "Suddenly, there was a loud crash behind the stage." The first crash came from somewhere on the stage; the second from a position even farther from the audience and beyond the stage.

CJ

Without context, I would take it as "behind the house".  A house has several rooms; which one constitutes the back one is a difficult decision.  "in back of the room" or "in back of the closet" are more easily understood as "in the back part of the ..."
CJ

Hi,

I see in back of  as a feature of AmE rather than BrE.

Clive

We sat back of the bus.

Is the above sentence acceptable?