Diplopia that is worse on looking down (reading, walking down stairs) is characteristic of a trochlear (CN IV) palsy: the superior oblique moves the eye down and out, so its weakness is felt most in downgaze. Abducens palsy gives horizontal diplopia with failure of abduction (medial squint); oculomotor palsy gives a down-and-out eye with ptosis and a dilated pupil; INO gives impaired adduction on lateral gaze.