Sink Drain - Why holes in sides?

I just installed a sink drain.
The drain has two retangular holes (about 1" by 1") in it just below the top surface of the sink and above the underside surface of the sink (above the undersink gasket)
What are the purpose of these holes? Seems like they just provide a cavity between the outersurface of the drain and the inner surface of the hole in the sink where the drain fits. Drain water must accumulate in this cavity, I would think?
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There should be an "overflow" slot in the sink up near the top edge. The water that goes through the overflow has to be able to get into the drain pipe, even if you have the plug in the basin, so it comes in the sides.
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MasterBlaster wrote:

Thanks, this sink does not have an overflow slot, so they serve no purpose on it. I guess any water that gets into these holes (gets caught in the cavity) will have to just evaporate out. This is a guest bathroom sink that is not used often
I could try filling the cavity, but I would have to take the plug assembly out again.
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snipped-for-privacy@yahoo.com wrote:

Those slots let the overflow water into the tailpipe. If you don't have an overflow, and the trap clogs, what's to stop water from backing up out of those holes?
When I installed a sink with no overflow (integral glass bowl and countertop) it came with it's own drain assembly that didn't have the overflow slots.
R
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Well it sounds like you have a sink with an overflow that has a tailstock that has no provision for that. So whoever put it in that way made a mistake. If it's not used very often it's probably not a problem. It would take water trapped in there a long time to evaporate. I have kids and this would definately be a problem. Richard
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snipped-for-privacy@yahoo.com wrote:

I'm not familiar with a vent under the sink. Usually the air is taken in at the sink overflow which is on the inside surface of the sink near the top as MasterBlaster posted, but I'm pretty sure that's what you're looking at - vent holes.
R
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That would be the over flow. Some sinks have them some do not. Your drain (actually the part called the tailstock) should have rectangular holes in it just below the stopper to allow the water that runs into the overflow to drain out. Richard
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replying to spudnuty, Mike wrote: I'm installing a drain and the water runs down the inside of the tailstock out the vent holes and down the outside of the tailstock onto the inside of the cabinet below. What am I doing wrong?
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On Friday, November 4, 2016 at 1:44:06 PM UTC-4, Mike wrote:

What "vent holes"?

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On Sat, 5 Nov 2016 11:25:30 -0700 (PDT), trader_4

Probably the place where you hook up a dishwasher. Sounds like the wrong tailpiece
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On Saturday, November 5, 2016 at 2:33:05 PM UTC-4, snipped-for-privacy@aol.com wrote:

But seems that would be a vent hole, not holes.... IDK, but something ain't right.
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replying to Mike, Lorrie Hedges wrote: Mike, I'm faced with the same thing. I have a custom sink bowl (no overflow) and the water is running out through that slot in the drain pipe. From what I have read on this site, some come with the slot and some don't. You need to replace the drain pipe with one that does not have that extra slot/hole for the overflow. I wish I had known this before. Practice, practice, practice..... Thanks for everyone's posts.
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