A means of referencing items with respect to time and ancestry such that an item without antecedents is designated as the first or n-th generation, and subsequent derivations are designated as n-1, n-2, etc.

• 从时间或世系的角度标记项的一种方式。没有前代的项可视为第一代或第n代，以后派生出来的称为n－1，n－2代等。