Ahnentafel is a German word meaning ancestor chart.
The person for whom the chart is prepared has the number one (1) in front of their name.
To find the father of a person, double their number. To find the mother, double their number and add one. Therefore, the parents of (1) would be (2) and (3). The parents of (3) would be (6) and (7). Fathers are always even numbers and mothers odd numbers.
By the way, if people repeat in a chart it is not an error. This happens when there are so many links between the various families.