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The picture shows the in-house gas valve for my outdoor grill. I want to make sure it is off as I never use it after purchasing the house. It has a square head and there is a nut on the other side. 

First questions: is it supposed to be square-headed by design or in fact the handle is missing?

Assume it is designed as a square head, I believe the following:

  1. The valve can be turned using a wrench. It is ON when one diagonal is parallel to the pipe (consequently the other diagonal is perpendicular to the pipe), and it is OFF when the sides of the squares are parallel/perpendicular. So the pic shows the valve is off. 
  2. Like many other valves, the nut on the other side is used for keeping the valve in place and preventing leaks.

Are my above belief correct? For item 1, it is possible that ON and OFF are indicated only by the diagonals. i.e., ON when one particular diagonal is parallel to pipe and OFF when the other diagonal is parallel to the pipe. Other positions are in the transition and not safe. 

  • Looks like a hole at the end, imagine for a screw to hold a handle on. It should stop turning at the on and the off points(full on and full off).
    – crip659
    Nov 20 '21 at 14:50

Looks like a plug valve that is missing a handle. So it has 2 'on' and 2 "off" positions in a 360 degree rotation. The handle would indicate the position, on or off. Likely the square flats line up with the plug bore, rather than the diagonals. However, there are many small valve designs and positions would need to be verified by operating the valve. And, yes , the nut keeps the plug in place; the plug could be tapered in either direction , or be straight.

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