Family tree

A family tree, also called a genealogy or a pedigree chart, is a chart representing family relationships in a conventional tree structure. More detailed family trees, used in medicine and social work, are known as genograms.

Genealogical data can be represented in several formats, for example, as a pedigree or ancestry chart. Family trees are often presented with the oldest generations at the top of the tree and the younger generations at the bottom. An ancestry chart, which is a tree showing the ancestors of an individual and not all members of a family, will more closely resemble a tree in shape, being wider at the top than at the bottom. In some ancestry charts, an individual appears on the left and his or her ancestors appear to the right. Conversely, a descendant chart, which depicts all the descendants of an individual, will be narrowest at the top. Beyond these formats, some family trees might include all members of a particular surname (e.g., male-line descendants). Yet another approach is to include all holders of a certain office, such as the Kings of Germany, which represents the reliance on marriage to link dynasties together.

The passage of time can also be included to illustrate ancestry and descent. A time scale is often used, expanding radially across the center, divided into decades. Children of the parent form branches around the center and their names are plotted in their birth year on the time scale. Spouses' names join children's names and nuclear families of parents and children branch off to grandchildren, and so on. Great-grandparents are often in the center to portray four or five generations, which reflect the natural growth pattern of a tree as seen from the top but sometimes there can be great-great-grandparents or more. In a descendant tree, living relatives are common on the outer branches and contemporary cousins appear adjacent to each other. Privacy should be considered when preparing a living family tree.^{[citation needed]}

The image of the tree probably originated with that of the Tree of Jesse in medieval art, used to illustrate the Genealogy of Christ in terms of a prophecy of Isaiah (Isaiah 11:1). Possibly the first non-biblical use, and the first to show full family relationships rather than a purely patrilineal scheme, was that involving family trees of the classical gods in Boccaccio's Genealogia Deorum Gentilium ("On the Genealogy of the Gods of the Gentiles"), whose first version dates to 1360.

In addition to familiar representations of family history and genealogy as a tree structure, there are other notable systems used to illustrate and document ancestry and descent.

An Ahnentafel (German for "ancestor table") is a genealogical numbering system for listing a person's direct ancestors in a fixed sequence of ascent:

and so on, back through the generations. Apart from the subject or proband, who can be male or female, all even-numbered persons are male, and all odd-numbered persons are female. In this scheme, the number of any person's father is double the person's number, and a person's mother is double the person's number plus one. This system can also be displayed as a tree:

A fan chart features a half circle chart with concentric rings: the subject is the inner circle, the second circle is divided in two (each side is one parent), the third circle is divided in four, and so forth. Fan charts depict paternal and maternal ancestors.