By default, new history states are added from the top down: oldest history states at the top and newest at the bottom. If you select a history state, all states that follow are dimmed and then deleted when you apply an edit. The new history state is then appended below the currently selected state. This method allows new histories to be added linearly.
However, there is a downside to working in a linear history mode: When you delete a state, all states that follow will also be deleted. If you prefer to hedge your bets and keep all the history states, you can force Photoshop to work in a nonlinear history mode. In this mode, when you select a history state and apply edits to the image, the new state will be added to the bottom of the list and the states that follow the targeted state won't be deleted. To set nonlinear history mode, choose History Options from the palette menu and then select Allow Non-Linear History (Figure 3-22).
TIP When working in linear history mode, you can use the Undo command to undo the last change and restore the eliminated states.
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