The "posterior semicircular canal" is a part of the vestibular system and detects rotations of the head in around the lateral axis, or in other words rotation in the sagittal plane.
The "posterior semicircular canal" is a part of the vestibular system and detects rotations of the head in the coronal plane. It is part of the bony labyrinth and is used by the vestibular system to detect rotations of the head in the sagittal plane.