How do you express relationships by marriage in one phrase?

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I understand that any blood relationship can be expressed either by direct-line terminology (son/daughter/father/mother/grandfather) or by cousin terminology (second cousin once removed). How do you express relationships by marriage with one phrase? There are mother-in-laws and father-in-laws and brother-in-laws and sister-in-laws, but how would you get the general relationship? For example, what is the relationship between my father's mother and my mother's mother?