A DBTL, or Double Blind Timeline, is an exercise used to create an alternate timeline. Participants take on the personas of someone from an alternate Earth and ask a what if question on an event that happened in OTL. As the participants discuss their thoughts from their alternate world perspective, the history of their timeline is slowly created.
It should be noted that in a DBTL that rarely do the people from the alternate world ever get our history correct. This emphasizes how difficult it really is to actually create a plausible alternate timeline when we are affected by our own experiences and bias.
An example of a DBTL in literature can be found in Philip K. Dick's The Man in the High Castle.