Molding and forming techniques can cause minor irregularities in the sizes or shapes of rigid glass and plastic bottles and jars, and some semi-rigid containers may also change their shape and volume slightly during the filling process. It is frequently more economical to use additional product in the oversized containers than it would be to more closely control the forming processes. Constant level filling techniques compensate for the minor changes and variations in the containers and produce the uniform appearance that is important in the sale of many products in ?see through? containers.