Working with History states in Photoshop

Normally, when you select a history state and then change the image, all states below the active state are deleted (or, more accurately, replaced by the current state). However, if you enable the Allow Non-Linear History option (from the History Options in the History palette menu), you may select a state, make a change to the image, and the change will be appended to the bottom of the History palette (instead of replacing all the states below the active state). You can even delete a state without losing any of the states below it! Note: The color of the horizontal lines between history states indicate their linearity. White dividers indicate linear states and black dividers indicate non-linear states. Another Note: Not only is a non-linear history very memory intensive, it can also be very confusing!