What does "proper left" mean?

The direction which is to the left from an object's perspective, the direction 90 degrees counter-clockwise from the front.

Proper left

noun

noun ·Rare ·Advanced level

Noun

1
The direction which is to the left from an object's perspective, the direction 90 degrees counter-clockwise from the front. uncountable
"The key in the lock served to open the proper left-hand door, while the proper right door was secured by a hook and eye of forged iron […]"

"The key in the lock served to open the proper left-hand door, while the proper right door was secured by a hook and eye of forged iron […]"

proper (in the sense of "its own") + left; from Latin proprius.