Standards and handbooks are mainly concerned with  plates. In these, the leading edge is sharp and free of burrs and the cylindrical section of the orifice is short, either because the entire plate is thin or because the downstream edge of the plate is bevelled. Exceptions include the  or  orifice, which has a fully rounded leading edge and no cylindrical section, and the  or  plate which has a bevelled leading edge and a very short cylindrical section. The orifices are normally concentric with the pipe (the  orifice is a specific exception) and circular (except in the specific case of the  or  orifice, in which the plate obstructs just a segment of the pipe). Standards and handbooks stipulate that the upstream surface of the plate is particularly flat and smooth. Sometimes a small drain or vent hole is drilled through the plate where it meets the pipe, to allow condensate or gas bubbles to pass along the pipe.