Most early brick houses with external cast iron air vents will have internal plaster vents inserted into the internal walls particularly in the main living areas. Usually they were inserted at a level to compliment the external vents however in many cases they were simply unpopular.Patterns were usually limited to the basic square configuration, however they can be found with variable slot combinations . To make vents more attractive manufacturers introduced intricate floral pasterns and other configurations. Plaster vents may be found in both single and double brick sizes.