It's code. This allows for clean out to be accessed from outside the home. The riser closest to the wall is the vent. The vent pipe is supposed to be at least 12" higher than the cleanouts. You can see the last cleanout in this case is a routing wye (not a 2 way cleanout). Also, the trap is not the same on both sides, inlet side has a bit more restriction than the outlet side (hard in - easy out).
edit: and no "Tees" ever in the trap.
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side view:
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Then, we do add some 2 way cleanouts occasionally down the line after a significant change in direction.
You're probably used to seeing the 3" cast iron 2-hole traps. Those are a real pita to get a snake thru.
here's a pic of the one we replaced yesterday...it was a handy man special...they used Tees, unreinforced fernco bands, and the lack of stone bedding was the death nail. The left side was the inlet, right side outlet. You can see how it sunk on the right side because their was no stone base (and there was a rain leader draining into the whole area).
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