Hi All, I read the Sunless Citadel in Tales From The Yawning Portal recently, & I notice that some traps use perception to find & some use investigation. How do you know which one applies to the trap.

Think about it as Perception is being able to notice things off hand, the table looks out of place, the bed sheets have been messed up, there is mark on the floor where the altar has been moved. Investigation is a more concerted effort to find things, looking for the mechanism that allows the altar to move, finding the loose floorboard under the table, finding the tiny blood stain in the bed sheets and maybe the poison dart.

Congratulations @Wickwire you have discovered one of D&D's most hotly contested subjects. And you are likely to get a vast array of different answers, most of which will be only partially correct but otherwise completely useless. Now, I am going to do something that probably no one else is going to do. I am not going to give you a partially correct but useless answer. Instead I am going to show you where you can find the information yourself so you can draw your own conclusions and come to your own totally correct and utterly useful answer. Ain't I swell? That was a rhetorical question by the way.

Anyway check out the PHB pages 177 and 178, including the sidebar on Hidden Objects. Then check out the DMG pp 103-104 & 120-123. You may find a few contradictory sentences but you can ignore them and remember specific beats general.

And this is why I am awesome because while everyone else will give a fish, most of which are small 3-day old carp, I will teach you how to fish so you can catch your own carp.

So I hope that I helped in my own small way.

Quote from TheGoodKingAzalin >>

Think about it as Perception is being able to notice things off hand, the table looks out of place, the bed sheets have been messed up, there is mark on the floor where the altar has been moved. Investigation is a more concerted effort to find things, looking for the mechanism that allows the altar to move, finding the loose floorboard under the table, finding the tiny blood stain in the bed sheets and maybe the poison dart.

Thanks for all the help adventurers but I got one more quest(ion) for you.When does passive Investigation come into play? The only place I even see about Passive Investigation is in the Observant feat. I can't find it anywhere else. Unless I am just missing it. 

Any passive score comes into play when the DM says so. Passive perception is particularly useful for hiding situations. Passive Investigation can be used for DM to determine basic information a character can deduct from observing the surroundings.

Okay I think I got it. Thanks All. I really appreciate the help.

Everyone has their own interpretation of how it works. For me, I've differentiated between the two by saying one is intuition and instinct, while the other is active intellectual engagement and critical analysis of evidence.

So perception is always passive (so I don't allow active perception rolls in my homebrew rules, you either notice something with instinct/intuition, or you don't) while investigation always requires the player to ask for a roll.

Perception: "Something looks off; it could be a trap!"

investigation: "Ah, a trap. If you move this lever here, this springs up. And let's see. If I do this, then the trap will be disarmed."

Basically an adventure will use perception to avoid a trap (loose tile on floor), and investigation to disarm a trap (trip wire on a gemstone).

Quote from Sillvva >>

I'd argue that Investigation would be used to know HOW to disarm the trap, but actually disarming it would be Sleight of Hand.