Flammable is to "Inflammable" as "Juncture" is to separation.
Juncture means to come together, or to consolidate, and seperation means to seperate. Flammable and Inflammable are opposites as Juncture and Seperation are opposites.
This is an analogy, a type of coparison which refers to a cognitive process of transferring information or meaning from a particular subject (the analog, or source) to another (the target), or a linguistic expression corresponding to such a process.