(+) and (-) tell you the direction in which an optically active substance rotates the plane of polarized light.
(+) is clockwise rotation (to the right).
(-) is counterclockwise rotation (to the left·
In the above image, the sample has rotated the polarization clockwise by 90°.
If the sample had been pulegone, we would have known it was (
If the rotation had been to the left (counterclockwise), the sample might have