I spent an embarrassing amount of time in a hardware aisle once, holding two outlets that cost about four dollars apart and looked, to me, identical. One had a small sideways notch off the left-hand slot. That notch turned out to be the difference between two different devices with two different ratings, and the reason I could not tell was that nobody had ever explained to me that an outlet's face is a label. It is not decoration, and it is not arbitrary. Every slot, every stamp, and every marking on the front of that plastic is telling you something specific about what the device is rated for and what is behind it in the wall.
The three openings, quickly
A standard receptacle has two vertical slots and a rounded opening below them. The wider slot on the left is the neutral, the narrower slot on the right is the hot, and the round opening is the equipment ground. The width difference is deliberate — it is what makes a two-blade plug polarized, so a lamp's shell ends up on the neutral side rather than the hot one. The round hole is the subject of the article on what a ground wire actually does, and the wiring behind all three is covered in the piece on what the colors of electrical wires mean. What matters here is that the shapes themselves are standardized, and the standard has names.
Why that sideways notch exists
The configurations are defined in ANSI/NEMA WD 6, the dimensional standard for wiring devices, which is where the naming most electricians use comes from. The ordinary household outlet is a 5-15R: three-wire grounding, 125 volts, 15 amperes, receptacle. The one with the sideways notch is a 5-20R, rated 20 amperes, and the notch turns the neutral slot into a T shape. That T exists so the receptacle can accept two different plugs. A normal 15-amp plug with two parallel blades goes in as usual, and a 20-amp plug — which has its neutral blade rotated 90 degrees, sideways — also fits. The rotated blade is a physical key. It means a 20-amp appliance cannot be plugged into a 15-amp receptacle by accident, because the sideways blade simply will not enter a straight slot.
A T slot does not promise a 20-amp circuit
Here is where the face stops telling the whole story. The rating stamped on a receptacle describes the device, not the wire feeding it or the breaker protecting it. Those are a separate pair, and the article on wire size and breaker size explains why the breaker is sized to the wire in the wall rather than to whatever gets plugged in. Someone can install a 20-amp receptacle on a circuit that is wired and breakered for 15, and the face will happily advertise a capacity that does not exist behind it. That combination is not permitted, but it is not rare either, and the outlet will not tell you. A T slot tells you the device is rated for 20 amps. Whether the circuit behind it is takes a look at the breaker and the wire gauge.
The reverse is normal and entirely legal
Plenty of homeowners find 15-amp outlets on a circuit protected by a 20-amp breaker and assume something is wrong. It usually is not. The rule turns on a distinction between a single receptacle and a duplex, and the North Carolina Office of the State Fire Marshal published a code interpretation on exactly this question. Its answer: a duplex receptacle counts as two receptacles, not one, so the rules for a single outlet on an individual branch circuit do not apply, and a 15-ampere duplex may be installed on a 20-ampere branch circuit under NEC Table 210.21(B)(3). What is not allowed is the lone receptacle case — one single receptacle by itself on its own circuit has to be rated no less than the circuit. That is why the outlet behind a dedicated appliance is worth a second look, and why the duplex in a bedroom is not.
The 12-amp number nobody mentions
A related table in the same code section, Table 210.21(B)(2), sets a maximum cord-and-plug-connected load of 12 amperes for a 15-ampere receptacle installed on a 20-ampere branch circuit. That number is easy to skip past and useful to keep. It means the 20-amp breaker is not an invitation to load any one receptacle to the hilt, and it explains why heavy single appliances tend to get outlets of their own rather than a share of a general-purpose circuit.
The stamps around the edges
- A testing-laboratory listing mark, most often UL, molded into the face or the mounting strap. An unlisted device is one nobody should be installing.
- The voltage and amperage rating, usually stamped small on the strap or the back rather than the front.
- TR for tamper resistant, the spring-shutter design covered in the piece on what those shutters are for.
- WR for weather resistant, intended for damp and wet locations rather than a spare bedroom.
- CO/ALR, which indicates a device rated for direct connection to aluminum branch-circuit wiring. If you see it, read the article on why aluminum wiring is a specialist's job before touching anything.
- On the back, small round holes beside the screw terminals — the push-in or backstab connections, which many electricians decline to use because they rely on a small spring rather than a clamped screw.
Two slots and no round hole
An outlet with only two slots and no ground opening is telling you something real about the wiring behind it: there is no equipment grounding conductor in that box. Swapping in a three-prong device does not create one, and doing so quietly hands every future occupant a ground pin that connects to nothing. The code recognizes one replacement path, which is a GFCI protecting the outlet, and it requires the result to be marked — NEC 406.4(D)(2) calls for the receptacle or its cover plate to read "GFCI Protected" and "No Equipment Ground," visible after installation. If you find a three-prong outlet in an old house carrying those stickers, nothing is wrong. If you find one carrying no stickers in a house that predates grounding, that is a question worth asking.
What to do with all this
Reading a face is free and takes a few seconds, and it is genuinely useful during the two moments homeowners need it: buying a house, and figuring out where the workshop tool or the window air conditioner should plug in. Walk the rooms once and note where the T slots are, where the two-slot outlets are, and whether anything carries a "No Equipment Ground" label. That list is worth more to an electrician than a description ever will be, because it tells them what the previous owners did without anyone opening a wall. Everything past reading — testing whether a ground is actually connected, changing a device, or confirming what breaker feeds what — is work for someone licensed, with a tester, on a circuit they have confirmed is dead.