In a recent thread concerning a washer stand pipe, the length of the trap arm was briefly discussed. One plumber stated that if the trap arm was too short (less than twice the diameter of the trap arm) then an 'S' trap would result. Another expert said that a trap arm could be as short as you need it to be as long as the trap is vented. I'm hoping this can be discussed a bit more. As a plumbing enthusiast, I've always been mindful to keep the length of the trap arm twice its diameter because I thought the weir needed some amount of air to prevent siphoning. However, it stands to reason that if a trap is vented, the length (or lack of it) of the trap arm becomes irrelevant. Or does it? Could the experts re-visit this to further illuminate the requirements? I'd like to understand this better. Thanks in advance.