In thin layer chromatography (TLC), the spots produced by programmed multiple development (PMD) are reproducibly and uniformly tight regardless of the initial top-to-bottom spread, and identical spots arrive at the same level on the TLC plate regardless of initial displacements of the spot origin from the plate edge. Both the spot reconcentration and the spot alignment that result from PMD can be explained by the same two mechanisms, one operating during solvent advance, the other during solvent removal. During solvent advance, the lower of two identical spots (or the lower edge of a single broad spot) begins moving before the upper spot (or edge). This solvent advance mechanism, common to all forms of unidimensional multiple development, decreases an original longitudinal separation Xi between two spots or molecules by the factor (1 — Rf)n, where n is the number of multiple developments. Experimental data for conventional multiple development agree with the prediction of the solvent advance mechanism; but for PMD, they show a further spot narrowing that must be attributed to the complementary solvent removal mechanism. During solvent removal, the lower spot (or edge) continues moving after the upper spot (or edge) has stopped. The solvent removal mechanism, unique to PMD, is the only one effective with Iow-Rf spots or molecules. Numerically unevaluable now, the mechanism predicts a decrease in Xi for each removal by the factor 1/(1 + R*), where R* expresses roughly the ratio of the spot velocity to the solvent front recession velocity.