One axis shows ages. Note the red “gaps" that separate age 17 from 18 and age 65 from 66. Another axis shows the years from 1900 to 2050. The third axis shows the number of people, ranging from fewer than one million (blue) to more than five million (red). Each white line on this axis represents one million people.
The black dot on all the views of the model locates teenagers in high school in 1995.