Differential distribution of solutes between two immiscible liquid phases—one held as a stationary film and the other moving as the eluent—is the classical definition of partition chromatography. Martin and coworkers realized the stationary liquid on paper or in a packed bed; each solute’s retardation is set by its partition coefficient and by the phase-volume ratio ^[1]. Filter-paper partition chromatography of sugars and paper partition of amino acids are planar embodiments of the same liquid–liquid distribution ^[2]. Gas–liquid partition chromatography extends the concept to a gaseous mobile phase in contact with a liquid film, which remains partition chromatography rather than adsorption (gas–solid) chromatography ^[3]. Conceptual boundaries exclude adsorption (liquid–solid) chromatography, ion-exchange chromatography, and size-exclusion chromatography. Essential features are two distinct liquid (or gas–liquid) phases, a support that holds the stationary liquid without dominating retention, and a reversible partition equilibrium. Countercurrent chromatography is a support-free relative; ordinary paper and column partition methods use a support.