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I'm a new DM, and I'm running through the Starter Kit adventure ( "Lost Mine of Phandelver "). At one point there was an opposing skill check for one of the PC's trying to sneak -- Stealth vs Perception. The player rolled a 20, and there was a bit of discussion about whether that was an automatic success -- similar to Critical Hits.

So, I'm wondering:

  1. Do all opposing skill checks automatically succeed on a natural 20 ( unless there's a tie )?
  2. If not all opposing skill checks automatically succeed on a natural 20, do any of them? If so, which ones?
  3. Besides saves & attack rolls, what treats a natural 20 as an automatic success?
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2  
Do not answer in comments. – mxyzplk Jan 12 at 12:57
up vote 14 down vote accepted

Do all opposing skill checks automatically succeed on a natural 20 ( unless there's a tie )?

No, but it's pretty hard for an opponent to beat a natural 20. The opposing roll only ever wins if its total check is higher despite the natural 20. This may happen, say, when a player has a negative modifier to a roll and the opponent rolls high and/or has high scores in the opposing roll.

In one of our games, my 8 strength Lore Bard beat our 18 strength fighter in an arm-wrestling, basically a Strength(Athletics) contest, by picking up Expertise in Athletics, throwing insults at him (Cutting Words Lore Bard feature), and cheating (casting Enhance Ability on myself).

If not all opposing skill checks automatically succeed on a natural 20, do any of them? If so, which ones?

Ability Checks don't have critical success unless house-ruled into your game but I'd be wary of this and that's another topic entirely.

Besides saves & attack rolls, what treats a natural 20 as an automatic success?

Only Attack Rolls treats a natural 20 as a critical hit, even if you roll a natural 20 on a Wisdom Saving Throw, you're still going to be frightened of that dragon if the total doesn't equal or surpass the Save DC.

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10  
Note: there are also many house rules on this. For example, in our group, a natural 20 guaranteed an exceptional event. Typically this meant you succeed, but there are plenty of other exceptional results that a clever DM can pull from. (an opposed arm wrestle between a wimp who roles a nat 20 and a strongman who beats him due to a decent roll + modifiers may still lose, but catch the attention of the damsel in the corner, or perhaps the strong man wins but pulls a muscle) – Cort Ammon Jan 12 at 6:15
    
@CortAmmon So something "exceptional" happens 5% of the time? Seems a but unexceptional to me. – corsiKa Jan 12 at 21:00
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@corsiKa It all depends on how much spice you want in your campaign! Due to utter coincidence, a p-value of 0.05 happens to be a recommended criteria for extraordinary events when rejecting hypotheses, according to Ronald Fisher, the guy who made p-values popular in science back in the early 1900's. I wonder what the odds of that coincidence are ;-) – Cort Ammon Jan 12 at 21:15

A natural 20 roll has no special meaning for an Ability or Skill Check, whether contested or not. It also has no meaning for a Saving Throw.

Critical Hits only apply to Attack Rolls. They are discussed in the Attack Rolls section in Chapter 9: Combat (page 73 of the Player's Basic Rules). They are not mentioned at all in Chapter 7: Using Ability Scores.

From chapter 9:

If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. In addition, the attack is a critical hit, as explained later in this chapter.

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Thanks! I don't have the PHB yet, and the basic rules that come with the Starter Kit are pretty sparse. – Sean Hagen Jan 12 at 3:53
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@SeanHagen You might like to download the free Basic Rules - they cover some areas more thoroughly than the Starter Kit. – Adeptus Jan 12 at 4:50

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