The vast majority of all noble families have agnatic succession, meaning both sons and daughters of a noble father are noble, but only grandchildren of sons (and not of daughters) are noble. Certainly in most families created noble by means of a letter patent issued by a Monarch, this is by far most common rule of succession.

Cognatic succession is, for practical genealogical purposes, only relevant in a few cases where it can be shown that this was indeed original form of noble succession of family, and there is an unbroken chain of succession from these (ancient) times down till today.

If we were to discuss succession to headship of a family, it would be necessary to distinguish between two variants of cognatic succession, true and false. True cognatic succession means firstborn child, whether a son or a daughter, has best claims. False cognatic succession would mean any son would inherit before a daughter regardless if he was younger, but in absence of sons oldest daughter would have best claims.

The kingdom of Sweden today has, according to its constitution, true cognatic succession, that is firstborn child of Monarch has best claims to succession to Throne. Some other Monarchies also have this system.

If, for genealogical purposes, you want to find out if a certain noble family as agnatic or cognatic succession, you need to either look at letter patent by which family was created noble, or, in case of original nobility, establish terms and conditions for noble succession in use at time and place where family was first recognised as noble. In most cases, family members living today will be able to tell you which form of succession they have.