In an RGB file, the distribution of tones in the Curves window goes from Black in the lower left corner
to White in the upper right, which puts lighter tones toward the top and darker tones toward the bottom.

The image here shows the output of a RAW file where adjustments were made only to ensure all tones fell
between the Black and White points and shows the tones being linearly ditributed between them since the ‘curve’
at this stage is a straight line. The image is flat, partly because the range of tones, as seen in the histogram
in the background of the Curves window, don't completely fill the space between the Black and White Points.

Click 'next' to see this corrected.