Pick up any cord from your lamp, toaster, or computer. Look closely at the flat prongs. There is a small hole near the tip of each one.
It’s a design quirk that has confused homeowners for decades. Is it for wiring? A factory defect?
No. The holes serve a mechanical purpose that keeps your power flowing safely.
To understand the holes, you have to understand the plug itself. In North America, you are likely dealing with Type A or Type B connectors. These are technically known as NEMA 1-15 and NEMA 5-15 standards.
Type A is the ungrounded version. Just two flat, parallel blades. Invented by Harvey Hubbell II in 1904, this design has dominated for a century. Type B adds a third, round or U-shaped pin. That pin grounds the device before power connects.
Both versions feature those mysterious holes in the flat prongs.
Why are they there?
The Grip Mechanism
Take apart an old outlet. Look inside the slots. You will see small metal bumps or detents.
These bumps slide into the holes on your plug’s prongs.
This is not just for show. It creates a friction lock. The outlet grips the plug more firmly. Without this, the weight of a heavy cord could slowly pull the plug loose. Gravity is a constant enemy in electrical connections.
Better contact is the second benefit. A snug fit ensures electricity flows without arcing or resistance. Loose plugs get hot. Hot plugs are fire hazards.
Older outlets used captive ball bearings and coil springs for this detent mechanism. Modern outlets use a simple bump and springy copper contacts. The principle remains the same: lock the plug in place.
Factory Seals and Safety Tags
Sometimes, you will see a plastic tie threaded through one of those holes.
This is a manufacturer’s safety lock.
Construction projects and industrial settings use “factory-sealed” or “locked-out” devices. The plastic band is attached to a warning tag.
The tag might say, “Do not plug in until maintenance is complete.” or “Grounding wire must be attached.”
The hole allows the manufacturer to physically prevent installation. You cannot plug the device in without cutting the tie. It ensures the user sees the instructions. It is a simple, low-tech safety protocol.
Material Savings
There is a third, less glamorous reason.
Metal costs money.
By punching out a small circle of metal from the end of the prong, manufacturers save on raw materials. It is a small saving per unit, but when you are producing millions of plugs, pennies add up.
It is a minor factor compared to safety, but it exists.
Common Plug Questions
What is a three-prong plug called?
It is a grounding receptacle. Type B plugs connect to a ground wire. This path diverts excess current away from your appliance and your body during a short circuit. It protects both.
What is a two-prong plug called?
Originally NEMA 1-15. Now commonly referred to as Type A. These are ungrounded. They are still very common in North and Central America, especially for low-power devices like phone chargers or lamp cords.
Which side is positive?
There is no positive.
We use Alternating Current (AC). The current flips direction constantly. Instead of positive and negative, you have “hot” and “neutral.”
The wider prong connects to the neutral wire. The smaller, thinner prong connects to the hot side of the circuit. If you reverse them, the device might still work. But the switch on your lamp or radio will then control the neutral line instead of the hot line. That is a shock hazard.
What happens if the third prong breaks off?
If you have a Type B plug and the ground pin snaps off, you lose your safety net.
Electricity will still flow. The device will turn on. But if there is an internal fault, the metal casing can become energized. There is no ground path to trip the breaker.
You become the path to the ground. This can ruin appliances. It can also shock or kill the user. Never file down a three-prong plug to fit a two-prong outlet. Use a proper adapter. Or better yet, upgrade the outlet.
DIY electrical work carries risk. Always turn off the breaker before touching wires. Verify power is off with a tester. And never ignore the ground pin.
