They are joined together by a key field or sometimes it is called a match field. The more you rely on relationships, the more it is a good idea to plan as much as possible before using them. Some folks like to employ a strategy in the defined names of their relationships. This helps them remember how things work when they have to look at the database after a few weeks, months or even years. A common on is to name the relationship with the name of the related file and some sort of code that reminds you what key fields you are using.
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