One of two general methods, the other being simple distillation, used to separate a substance or a mixture of substances from a solution through vaporization. Distillation usually involves boiling a liquid and condensing the vapour that forms in a still. In simple distillation, all the distillate is removed from the still after collection. In rectification, part of the distillate flows back into the still. This portion comes into contact with the vapour being condensed and enriches it. Rectification can also be undertaken using large towers (fractionating columns). As the mixture to be separated is heated, its vapours rise through these columns. Substances that boil at the lowest temperatures form the first fractions. Their vapours rise highest and are carried off by pipes near the tops of the fractionating columns. Separate pipes carry off different fractions at various levels. Reflux (return) of some distillate to the columns produces the most efficient conditions for this method of distillation. Rectification can be carried out with a continuous feed of liquid. During manufacture of vodka, by-products of distillation, such as methanol, are removed from the distillate by rectification using a continuous still.