A flat layer of stationary phase, developed by capillary-driven or forced flow of a mobile phase in two dimensions of a plane, defines planar chromatography. Paper chromatography and thin-layer chromatography are the principal members; the solute zones migrate as spots or bands whose positions are described by Rf or related retardation factors ^[1]. After development, the plane can be interrogated by densitometry, bioassay, or direct mass spectrometry without eluting the zones from a column ^[2]. Conceptual boundaries exclude column liquid chromatography, capillary chromatography, and electrophoretic separations on a plane that use an electric field as the sole transport force. Essential features are a planar bed, a developing solvent, and a two-dimensional spatial record of the separated zones. Hyphenation of the developed plate to ambient ionization mass spectrometry treats the planar chromatogram as the sample surface, which does not alter the chromatographic definition ^[3]. Planar geometry also permits multidimensional development in different solvent directions on a single plate, a configuration unavailable in a conventional one-dimensional column ^[4].