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I see that you live in the US, so my answer is for the US.SIMPLE ANSWERThe extra notch on a 20 amp receptacle is to allow a 20 amp plug to be inserted into the 20 amp receptacle.20 Amp PlugA 20 amp plug will not fit into a 15 amp receptacle.This feature prevents a 20 amp plug from inadvertently being inserted into a 15 amp receptacle (that might be protected by a 15 amp fuse or circuit breaker) as doing so might blow the fuse or trip the circuit breaker. Therefore this feature helps minimize inadvertent circuit overloading that would result in circuit breaker tripping.(These plugs are not common in residential applications, so you may not have seen one. One residential application is for larger window unit air conditioners. They are not uncommon in commercial kitchens and other commercial applications, and motels and the like often use them for air conditioner/heating units.)THE DETAILS20 Amp Receptacles20 amp, 125 volt receptacles (they are technically called receptacles, but many people call them outlets) have an extra notch in them.15 and 20 Amp, 125 Volt , Grounding-Type Duplex ReceptaclesOn the receptacles the shorter slot is for the hot side (typically black, sometimes red wire connected to the brass colored screw terminal). The longer slot is for the neutral side (white wire to silver-colored screw terminal).This is so that a 15 amp, 125 volt polarized non-grounding plug can not be inserted in the reverse direction where the neutral for the appliance could be inadvertently and unsafely connected to the hot wire from the receptacle.15 Amp, 125 Volt Polarized, Non-Grounding Type PlugWhy the Extra Notch in a 20 Amp, 125 Volt ReceptacleThe extra notch in the 20 amp receptacle is there to prevent 20 amp plugs from being able to be inserted into 15 amp receptacles (which could potentially overload the 15 amp circuit, which is not a good thing).A 20 amp, 125 volt plug looks like that shown below. They are not very common in the US in residential applications, but they are not uncommon in commercial applications (commercial kitchens, etc.).20 amp, 125 Volt Grounding-Type Plug15 Amp, 125 Volt Non-Grounding and Grounding Type PlugsAs you can see, a 15 amp, 125 volt plug can be inserted into either a 15 amp or a 20 amp, 125 volt receptacle, whereas a 20 amp, 125 volt plug can only be inserted into a 20 amp, 125 volt receptacle.NEMA Plug & Receptacle StandardsReceptacle (and plug) types for the US are defined in NEMA (National Electrical Manufacturers Association) standards. Charts are available for these, two of which are shown below. (There is one for non-locking type plugs/receptacles, one for locking type plugs and receptacles.)15 and 20 Amp, 125 volt ReceptaclesIn the US, the National Electrical Code (NEC) is the electrical “installation code used in all 50 states and all U.S. territories …” [reference 2020 NEC Preface].In the US per the NEC, 20 amp receptacles are to be used on 20 amp circuits (circuits protected by 20 amp overcurrent protection devices - a fuse or a circuit breaker, with appropriate wiring).15 amp receptacles are used on 15 amp circuits. A 20 amp receptacle may not be used on a 15 amp circuit.On a 20 amp circuit, if there is more than one 15 amp receptacle, 15 amp receptacles may be used. A duplex receptacle is two receptacles, so a duplex 15 amp receptacle can be used on a 20 amp circuit. A single 15 amp receptacle may be used if there is more than one 15 amp receptacle on the circuit.15 Amp, 125 Volt Grounding-Type Receptacle15 Amp, 125 Volt, Grounding-Type Duplex Receptacle15 and 20 Amp CircuitsA 15 amp circuit is one protected by a 15 amp overcurrent protective device (fuse or overload). Most commonly 15 amp circuits use building wire of 14 gauge, but larger wire sizes may be used for long runs for voltage drop reasons. Most commonly 12 gauge is used for building wire for 20 amp circuits.I can supply more information including references in the NEC if anyone would like, as many people seem to be confused about the proper application of 15 and 20 amp, 125 volt receptacles in the US. For example, many untrained people believe a 20 amp receptacle may be installed on a 15 amp circuit, which is not allowed per the NEC. - for good reasons (so as to not overload a 15 amp circuit).There is so much more that can be said on items related to the topic (tamper-resistant receptacles, weather-resistant receptacles, faceplates, GFCIs, AFCIs, split-wired receptacles, etc.) which I will add if and as additional questions arise.

Well, let's see:Nothing happens. I’m still alive and posting on Quora.Why? Let’s measure the current. (My right finger is on the negative meter probe, and the meter is set to the 2 mA range)0.026 mA; that's 26 micro amps, that’s 12V × 26µA = 312 micro watts of power being dissipated in my body (as heat).Nothing is going to get burned with 312 micro watts.You may wonder why a battery capable of providing 1000 amps, only provides 26 micro amps. Because of resistance, as others have said. Skin resistance is very high, and current is voltage divided by resistance.Now, if I drop a wrench (with essentially 0 ohms of resistance, Yes I know it’s not really 0 ohms, but close enough for our purposes) across the battery, the current would be 12(volts) divided by 0(ohms), or infinite current. Of course, that’s not possible, but the battery will provide everything it can. This will probably be about 1000 amps. So 12(volts) times 1000(amps) is 12,000 watts. That’s a lot of power, and will easily melt parts of the wrench. A metal watch band or ring can get hot enough to melt, and seriously burn you if the battery somehow contacts it in two places.So, your answer is nothing will happen because your skin resistance is way too high to conduct any significant current at 12 volts.For the adventurous out there put a 9 volt battery on your finger and you will feel nothing. Put the same battery on your tongue and it will hurt A LOT. (Don’t say I didn’t warn you). That is because a wet tongue has much lower resistance than skin.Edited:Wow, almost 1000 upvotes. Thank you.I am adding a picture of what can happen to a wrench.

Well, they will probably plug you into a VOX AC30 an Orange Crush or a Fender Twin to “demo” it for you. Mistake.You will then be auditioning the sound of the most awesome amp as much as (if not more than) the sound of the guitar. But you will not take that amp home, and as we all know, the guitar is only a third of the sound. The amp is a third and your technique is too.If possible take along your own practice amp (call them maybe, first “hey buddy, can I bring my shitola solid state amp?”) or ask to be demoed through an amp as close to your own home amp in wattage, character and speaker size. If it sounds good through a no-name crappy amp, to it will sound good anywhere.Beware them saying “try this dirt pedal dude” or “run it through this boss compressor and hear it sing”. You are not buying those, you are buying the guitar, focus on that. You can’t take their store tone home but it’s part of effective sales.Watch out for a warped neck, dull strings, a too high nut etc, but essentially, check that what you hear in the store is what you will get at home.