A distillation column that is equipped with bubble cap trays, also known as bubble trays. These are flat perforated plates with short risers covered with inverted caps over the top. The caps have slots and allow vapour to rise through a liquid on the tray but prevent the liquid from draining back. This type of distillation column was common before the 1960s prior to the introduction of much cheaper sieve plate column and valve tray designs.