Unstable identifiers are responsible for a significant share of the maintenance burden. Identifiers are unstable when they contain information, because virtually any information about the thing identified is subject to change. In contrast, stable identifiers are information-free, conforming to the principle of strict uniqueness discussed below. Stable identifiers are an essential component of a stable information structure, and they play an important part in reducing the maintenance burden. It is essential that the identifiers (primary keys) used in a system not be subject to modification as the requirements of the system change.