Is this an S-Trap on the drain line, or is it some other kind of trap?
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I’ll say yes it is. That’s what my union plumbing teacher taught. Now for other opinions……
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It’s the most complicated s-trap configuration that I’ve ever seen. Thanks for the feedback, @mwilles
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Is there such a thing as a loop-d-loop trap? 
Maybe you found one…
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Right! Maybe a box-loop or square-loop trap? I just made that up.
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Yeah, I can’t recall a more labor-intensive yet still wrong installation. If you don’t care about doing it right there was definitely a shorter road. 
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It looks like the other side of the joist may have worked.
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I think that the original installation had the trap on the back side of the joist through a notch on top of the joist. Whoever repaired it lopped off the rest of the joist and brought the trap below the joist.
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Short answer, not a S-trap.
Longer answer, probably not an approved trap because arguably not self-scouring with the connections.
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My instructor also. Spinning the sections doesn’t make an s-trap okay. I actually wonder if it’s more likely to siphon the trap like that? Does the lateral movement of the water add centrifugal force?
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Thanks for the nuanced analysis, Lon. Self-scouring is a new term to me. I had to look that one up, but it now makes sense.
Wow! It drains down, and then back up again. There should be plenty of stagnant water in that configuration.
I would say yes. The trap arm appears to be long enough, but there is no vent, which would also cause this to act as an s-trap.
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Not an S-trap.
S-traps are a bit like how some Supreme Court justice explained pornography, " I can’t define it, but I know it when I see it." The problem with the justice’s definition, is the definition is in the eye of the beholder. Several of us don’t see an S-trap and some do. I say it is not an S-trap because the exit from the weir has been extended more than 2X the diameter of the pipe. That makes it far more difficult to siphon dry. I say it is almost certainly an illegal trap because the trap must be smooth on the interior (self-scouring). The pipe connections do not make for a smooth interior. The photo does not show us the proximity of the vent pipe.
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As I said, the trap arm appears long enough, and if that is all you go by, then correct, it’s not an s-trap…
However, the lack of vent AFTER the trap also causes it to function as an s-trap and will siphon it dry. If this line does have a vent, it is way before the trap in question.
P3105.1 Distance of trap from vent:
Each fixture trap shall have a protecting vent located so that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table P3105.1.
Exception: The developed length of the fixture drain from the trap weir to the vent fitting for self-siphoning fixtures, such as water closets, shall not be limited.
❖ The distance from a trap to its vent is limited to reduce the possibility of the trap self-siphoning, which is the siphoning caused by the discharge from the fixture the trap serves. Excessive distance results in excessive fall resulting from the pipe slope and this could cause the drain to resemble an S trap in function. S traps and piping arrangements similar to S traps are prohibited because of the tendency to self-siphon the trap [see Commentary Figures P3105.1(1) and P3105.1(2)].
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Although technically correct… I seriously doubt that “Trap” will ever get “siphoned dry”, if for no other reason than the sheer volume of liquid it holds! That trap is oversized and is prone to
clogging/blockages, not necessarily siphoning!
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For a lack of a more specific classification, I called it an S Trap. However, I would think that the size of the trap and its excessive bends would make it more prone to clogging and blockages than it would be to siphoning. Just my guess, though. From the comments above, we all seem to recognize that the configuration is wrong and prone to problems.
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