A plate or tray used in a distillation column in which liquid flows enter from one side and flow across and out of the other (see Fig. 15). In a distillation column the liquid is retained on the plate and prevented from descending through the perforations by the rate of upflow of vapour, bubble caps, or valve seals. A weir is used to control the depth on the plate over which the liquid descends to the plate below via the downcomer.