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I'm connecting the drains for a double basin sink something like the one in this picture. My question is, do I need any slope in the horizontal line between the two basins or after the p-trap in the section leading to the vertical?

enter image description here

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You do. You want at least 1/4" per foot to keep liquids flowing and carrying any sediment or waste into your drain line. It shouldn't be hard to accomplish given your setup.
Edit
To adjust height you need to loosen the nut on the right tailpiece and raise it up on the slip joint. You may have to trim a little off the tailpiece to do that. Do the same for the lower tailpiece on the left below the inlet and above the trap. Slip joints will typically have enough play to allow you to make these type of adjustments but sometimes they require trimming.
One tip - It's often easier to make this type of adjustment if you slightly loosen all of the plastic nuts first to give you a little play when setting the slope. Just don't forget to retighten afterward and check for leaks.

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  • Thank you. Is this accomplished simply using the flex of the pvc? The pieces themselves seem to be cut without any particular angle in mind.
    – billynoah
    Apr 27 '20 at 11:34
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    Loosen the nut on the right sink drainpipe and raise the pipe - if the tailpiece is too long to allow raising it on the slip joint, remove the drain and trim the tailpiece shorter, then reassemble. Do the same thing on the inlet side of the trap (again, if need be, trim the tailpiece shorter.) Those joints ARE slip joints precisely so you can make these adjustments. Brand-new tailpieces are long, so you can trim them shorter if need be.
    – Ecnerwal
    Apr 27 '20 at 13:03
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Yes it needs to be slightly sloped.

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    While technically correct, it is more in the spirit of this site to had more information. There's not a lot to add in this case but perhaps at least note how much slope.
    – Ack
    Apr 27 '20 at 7:08

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