From Draw Like da Vinci, p. 37: The natural inclination is to shade on an angle (obliquely). Left-handers and right-handers shade in opposite directions. The left-hander's downward stroke is from top left to bottom right, and their upward stroke is fromm bottom right to top left. The right-hander shades the other way around – the downward stroke is from top right to bottom left, and upward is from bottom left to top right.