… Multiple simultaneous referents:

ref points to a link that links the persons of interest

presidents Adams were born in Massachusetts
Adams, John Adams, John Quincy

If you have two or more people who are frequently grouped together, you may also use the link element to group them. If you define the group as one link within a linkGrp, you can then use ref to point to the xml:id of the link, rather than to the two separate entries in the personography.

Contextual Nuances, slide 8 of 11