"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
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