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The 3D-adjustometer. You can select how deep of a 3D field of view you'd like by adjusting this slider up or down.
The top display of the 3DS reportedly produces 3D images through the use of parallax. A layer in the top LCD responds to adjustments in the 3D slider and changes the image slightly between each of your two eyes. When it is viewed from the recommended distance, the image appears to be 3D.
Nintendo is no stranger to 3D gaming, considering its past experience with the ridiculously cool and underrated Virtual Boy that uses slightly different means to achieve a similar 3D effect.