This trope is about characters who wouldn't fit together now, for whatever reason, but about whom there is a general consensus in the fandom - even in fics focused on another pairing - that they used to date/have slept together at some point in the past. This does not apply to Canon exes. May be secondary to another pairing, not considered a threat but treated rather condescendingly, as in the Merlin examples below. Alternatively, it may be a popular ship which assumes that the Foe Yay they have now is due to a horrible, horrible break-up. Compare Star Boarding.