It is helpful to use a numbering scheme when referring to the ancestral families in your family tree. Referring to a family by the names of the spouses is problematic. Their names might change as new information about them is discovered. A number is also easier to reference in a filing scheme for paper documents or index cards, as well as for computer files and Internet web pages.
Here is a method for numbering families (not individuals) for your direct-line ancestry. First, notice that an ancestor or pedigree chart is actually a “binary tree,” meaning that each node, or family, in the tree always has exactly two ancestor nodes. Each of the two nodes can therefore be assigned a unique number. This includes even the “missing” nodes, or families that you have yet to discover.