Record linkage is the technology necessary to discover matching records. To bring about a match it is necessary by definition that the records relate to the same entity. Problems arise when the entities are of the same kind, but are not identified in the same way. In a census schedule, for example, the individuals may be identified by name, sex, age, birth place, marital status, and relationship to the head of the household. In contrast, in a marriage return, the individuals may have a name, sex, and age. If we are to link records of individuals, we need to make their identifiers in these two types of records compatible for comparison. Name and sex are often rather directly comparable (more about this later). Ages in conjunction with the date of the event (census viz. marriage) will transform into the individual's birth date. Further, if we are to link records of the families, it will be necessary to transform the husband-wife relationship in the census to the groom-bride relationship implied by the marriage. There would be a requirement that the census event supporting this linkage chronologically follow the marriage event; the latter of these two family events begins the family. In addition, their successful individual linkage would be another precondition on the family linkage. In other words the same principals on both records generally* implies that they form the same family.
[*“generally” since chronology is also important: the remarriage of an individual with an ex-spouse would constitute a different marriage of the same individuals!]