The most common type of valve seen for valve attachments is the rotary valve. Some trombones have piston valves used instead of rotary valves for valve attachments, but it is very rare and is today considered unconventional. Many variations of the rotary valve have been invented in the past half-century, such as the Thayer valve (or axial flow valve), the Hagmann valve, the Greenhoe rotor, and several others, all of which were designed to give the trombone a more open, free sound than a conventional rotary valve would allow due to the 90° bend in most conventional rotary valve designs. Many of these new trombone valve designs have enjoyed great success on the market, but the standard rotary valve remains the most common for trombone valve attachments.