Sometimes, after meeting a friend's significant other, someone will observe that the man or woman in question is "the marrying type." Others around will nod wisely and pensively sip their drinks. (I imagine this sort of thing happens in a dimly lit bar, where the friends have convened to imbibe and pass judgment.) What exactly identifies this person as the marrying type is unclear-maybe it's a certain sparkle in their eye, or maybe they have helpfully tattooed a dotted outline on their left ring finger where a wedding ring might go.