The Trieste J had a built in reserve mechanism. Many divers liked this option so they did not have to have J valve tank vales on their tanks as the were more prone to leak. The Trieste is described by some divers as the the best that Voit ever made. I think this is greatly inaccurate. While it is an elegant regulator and very unique I do not find it comparable to a US Divers Royal or Mistral. I think that the small diaphragm casues a mechanical disadvantage as compared to other vintage double hose regulators. I have dived many Triestes and do like them however. One very nice feature of the Trieste regulators is the low and high pressure ports.