A graduated bob is an A-line with stacked layers in the back. The back is also more curved (in a horseshoe shape) rather than a hard angle. The shape of the layers should follow the angle of the perimeter of the haircut, getting longer towards the front. This type of haircut is the most common type of bob I see right now. The amount of stacking can vary. The front is typically a little shorter than the front of an A-line haircut.