Context: DIY shelf with industrial water pipes. (NOT plumbing! -- guess such construct does not make much sense there.)

Consider e.g. this example, if you look at it from the side, it has some "closed loops", where I define closed loop as: T-connector #1 -- pipe A -- T-connector #2 -- pipe B -- T-connector #3 -- pipe C -- T connector #4 -- pipe D -- T-connector #1. See also schema below:

         A
#1 ├------------┤#2
   |            |
 D |            | B
   |            |
#4 ├------------┤#3
         C

Question is: what is the way of building such a "loop"?

Of course, it is trivial to build one semi-loop with only 3 pipes (T-connector #1, pipe A, T-connector #2, pipe B, T-connector #3, pipe C, T-connector #4). But then, how do you insert pipe D?

Another idea would be to build first the sections T#1 - D - T#4 and T#2 - B - T#3 separately. And then somehow screw in pipes A and C simultaneously. However, (unless I'm mistaken) if you tighten pipe A in connector #1, then you are loosening it in connector #2 and vice-versa.

So it seems impossible to build such a loop, but obviously there is a way, as the example in the link (and many others) demonstrate.

Context: DIY shelf with steel threaded pipes and fittings. (NOT plumbing! -- I guess such a construct does not make much sense there.)

Consider e.g. this example, if you look at it from the side, it has some "closed loops", where I define closed loop as: T-connector #1 -- pipe A -- T-connector #2 -- pipe B -- T-connector #3 -- pipe C -- T connector #4 -- pipe D -- T-connector #1. See also schema below:

         A
#1 ├------------┤#2
   |            |
 D |            | B
   |            |
#4 ├------------┤#3
         C

Question is: what is the way of building such a "loop"?

Of course, it is trivial to build one semi-loop with only 3 pipes (T-connector #1, pipe A, T-connector #2, pipe B, T-connector #3, pipe C, T-connector #4). But then, how do you insert pipe D?

Another idea would be to build first the sections T#1 - D - T#4 and T#2 - B - T#3 separately. And then somehow screw in pipes A and C simultaneously. However, (unless I'm mistaken) if you tighten pipe A in connector #1, then you are loosening it in connector #2 and vice-versa.

So it seems impossible to build such a loop, but obviously there is a way, as the example in the link (and many others) demonstrate.