Onion skinning is Flash's answer to light tables. Animators often use light tables so they can see where exactly the drawing is on the previous pieces of paper, as the light makes the paper transparent. They can then draw the the character on the new piece of paper, knowing exactly where it should be.

In Flash, the equivalent is the Onion Skin button, which is located just underneath the timeline along with 4 other buttons.





In this example, we have a 15 frame animation of a stick man moving from the left hand side of the stage, to the right hand side of the stage. In each keyframe, the stick man has been moved slightly to the right. To enable the onion skin. Click the button underneath the timeline (as pictured above)

You should notice two draggable handles within your timeline. Think of them as brackets. All the frames within these two brackets will be shown onion skinned. You can drag the left or right hand bracket with your mouse. If you want all your frames onion skinned, drag it to the right hand side.





With that done, your stage should look similar to this:





In the above image, the last frame has been selected. It is perfectly visible. The frames before that are also visible, but progressively fade out. This technique is great for simple animations where you have edited each frame yourself. There is also, however, the option to select Onion Skin Outlines. The button for which is on the right hand side of the regular Onion Skin button. WIth this selected, the same thing happens, only this time, you only see the outines of the character on the previous and future frames. The frame that you have selected is shown in full color.

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