The mobile phase composition does not have to remain constant. A separation in which the mobile phase composition is changed during the separation process is described as a "gradient elution". One example is a gradient starting at 10% methanol and ending at 90% methanol after 20 minutes. The two components of the mobile phase are typically termed "A" and "B"; "A" is the "weak" solvent which allows the solute to elute only slowly, while "B" is the "strong" solvent which rapidly elutes the solutes from the column. In reversed-phase chromatography, solvent "A" is often water or an aqueous buffer, while "B" is an organic solvent miscible with water, such as acetonitrile, methanol, THF, or isopropanol.