A stage-by-stage calculation used for solving multicomponent distillation problems. The calculation operates from both ends of the column and works towards the middle. The calculations are required to ‘mesh’ somewhere in the column, (p. 220) which is usually at a feed stage. For more than one feed a choice of mesh points is made for each component and, if the components vary widely in volatility, the same mesh point cannot be used for all components if serious numerical difficulties are to be avoided. The procedure is subject to large truncation-error build-up where components differ widely in volatility. Arbitrary procedures are also needed for non-distributed components whose concentration in one of the product streams is smaller than the smallest number carried by the computer. The concentrations for these components do not naturally take on non-zero values at the proper point as the calculations proceed through the column.