In the rental house I'm in now, the T&P release valve on the water heater is connected with copper discharge tubing which exits the garage. See this picture 
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The right-side of the pipe seems to be simply threaded into the T&P discharge valve with some plumber's tape, but the left side is connected to an (otherwise soldered) series of copper pipes by this nut looking thing. I'm curious how this arrangement was likely created, given the fact that these pipes are obviously not flexible connectors. Was it likely that the left side was connected first, and then the water heater was positioned into place exactly so the threading on the right worked? Or was this horizontal copper pipe screwed into the T&P release valve first (allowing the water heater to be positioned semi-independently) and then that nut-like thing somehow helping the copper piping to be connected to the elbow in a way which helped the fact that otherwise the piping would have to have been seemingly cut to the exact perfect length to work. Perhaps the nut allows for some fitting to be placed there and then the horizontal tube was soldered into place while connected to the water heater?
I don't need to re-engineer this or do something similar, but I'm curious how this connection would have likely been made with rigid piping. What is this nut? How would the left-hand side likely been connected?
Best Answer
That fitting is called a union, it was installed to allow for the T&P and/or the entire water heater to be replaced without needing to cut and resolder the relief valve discharge line.
As for the order of operations during initial installation, there really is no way to tell for sure. Assuming the discharge line exits the utility area where the heater is located then a very likely scenario is that the installer may have "roughed in" the discharge line prior to the closing of the walls with sheetrock and then simply piped to it from the T&P valve once he placed the water heater in its final location. In this case it is likely that they would have begun on the right with the male adapter going into the T&P valve, then measured and cut the pieces and added the fittings, working to the left until they made connection with the previously installed discharge piping. It is also possible that they piped it the opposite direction, from left to right, but personally I think its easier to go right to left in this case. The union isn't required either by code or by the fact that it is rigid copper piping, it is simply there to ease the replacement of the tank itself and the relief valve.