You generally need neither a special tool to turn on or off the main gas valve. They are usually a very simple and reliable on/off valve similar to a ball valve. A quarter turn is all that it needed. The "bar" (spoken earlier in this thead) is usually running in the same direction as the pipe. You will use this "bar" to put your wrenchn on to turn the vale's iner workings. This is in the on position, to turn it off you take your cresent wrench ( which a lot of people now have chained to the main gas supply pipe ) and thurn it one quarter turn until the "bar" now runs at a 90 degree to the supply pipe. <br>You can just as easy reverse the process to turn it back on. BUT DO NOT. The reason is that when the gas is shut off it extinguishes all the pilot lights in the stove, hot water heater and furnace, etc. These need to be relit by a knowlegable person or preferably by a liecenced gas fitter. After the big one is not the time to try and learn this skill and do it for the first time. <br><br>So. Go ahead and get a large pipe or cresent wench and attach it to or near the gas service. Either get a licenced gas fitter to take you through the process beforhand or just shut it off when the time comes and wait for the right help to turn it on and relight the appliances when their time is available