Cadency is any systematic way of distinguishing similar coats of arms belonging to members of the same family. It is a kind of differencing.

Cadency is necessary because in most heraldic systems a given design may be owned by only one person (or, in some cases, one man) at once. However, because heraldic designs may be inherited, the arms of members of a family will usually be similar to the the arms used by its oldest surviving member (called the "plain coat"). They are formed by adding marks called brisures, similar to charges but smaller. Brisures are generally exempt from the law of tincture.

The English system of cadency involves the addition of these brisures to the plain coat:

• for the first son, a label of three points (a horizontal line with three tags hanging down)-- this label is removed on the death of the father, and the son inherits the plain coat;
• for the second son, a crescent (the points upward, as is conventional in heraldry);
• for the third son, a mullet (a five-pointed star);
• for the fourth son, a martlet (a kind of bird);
• for the fifth son, an annulet (a ring);
• for the sixth son, a fleur-de-lys;
• for the seventh son, a rose;
• for the eighth son, a cross moline;
• for the ninth son, a double quatrefoil.

Daughters have no special brisures, and use their father's arms on a lozenge. This is because English heraldry has no requirement that women's arms are unique.

Arms are generally the property of their owner from birth.

The eldest son of an eldest son uses a label of five points. Other grandchildren combine the brisure of their father with the relevant brisure of their own, which often leads to confusion.

The rules for members of the Royal Family are substantially different. At birth, they have no arms. At some point during their lives, generally during their teenage years, they may be granted arms. These will always be the arms of dominion of the Sovereign with a label argent for difference; the label may have three or five points. Traditionally, a stock series of symbols (cross, heart, anchor, fleur-de-lys, etc.) has been placed on the points of the label to ensure that such arms differ. However, a recent innovation, used for Princes William and Harry, is to use a symbol representing their mother: the princes' arms use a scallop shell, symbol of the Spencer family, for difference.

It is often said that labels argent are a peculiarly royal symbol, and that eldest sons outside the royal family should use labels of a different colour, usually gules.

Scottish cadency involves a complicated system of bordures of different tinctures. It is far more precise than the English system.

In addition, because of the Scottish clan system, only one bearer of any given surname may bear plain arms. All other bearers of that name, even if unrelated, must have arms which reference these plain arms somehow. This is quite unlike the English system, in which the surname of an armiger is generally irrelevant.

Canadian cadency generally follows the English system. However, since in Canadian heraldry a person's arms must be unique regardless of their gender, Canada has developed a series of brisures for daughters:

• for the first daughter, a heart;
• for the second daughter, an ermine spot;
• for the third daughter, a snowflake;
• for the fourth daughter, a fir twig;
• for the fifth daughter, a chess rook;
• for the sixth daughter, a scallop shell;
• for the seventh daughter, a harp;
• for the eighth daughter, a buckle;
• for the ninth daughter, a claricord.