When vapor moves up through a fractionating column, it goes through multiple cycles of evaporation and condensation. These cycles happen because of the many indentations or surfaces inside the column. The vapors temporarily condense on these surfaces before vaporizing again due to the heat from the heating mantle. Every cycle of evaporation and condensation is called a theoretical plate. With each cycle, the amount of the lower boiling point component in the distillate increases.

The number of theoretical plates needed is equivalent to the number of simple distillation cycles we would have to do to reach the same separation. Using theoretical plates is a more practical way of performing more than one cycle of distillation without using large amounts of equipment and space in the lab.