Histograms 

high key scene 

  

Most of the pixels fall to the right. there are no pixels that are clipping (that won't print with detail.)

Low Key Scene

     

Most of the pixels fall to the left of the histogram. There are no pixels that won't print with detail.

Varied Tones

   

Most of the pixels fall in the middle of the image but leans more towards the left side. There no pixels that won't print with detail. This histogram shows the most dynamic range.