If you look closely at almost any two- or three-prong plug sold in North America, you’ll notice a small hole near the tip of each flat blade. These holes aren’t decoration—they date back to the earliest days of household electricity and still serve useful purposes today.
A Brief History
Harvey Hubbell, whose 1913 patents shaped the modern Type A/B plug and receptacle, added holes so spring tabs inside the outlet could “click” into place and keep the plug from slipping out of the wall Third Taxing District. By the 1920s this retention feature became common, giving consumers a firmer connection before grounding blades were even standard.
Over time manufacturers also discovered two side benefits:
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Manufacturing alignment – An alignment rod through the holes keeps blades straight during automated molding Laughing Squid.
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Oxide scraping – Some receptacles use the edge of the hole to wipe oxidation off the blade for lower resistance Hacker News.
Why the Holes Still Matter
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Secure Grip in Standard Outlets
Most duplex receptacles still have tiny “detent bumps” that snap into the holes, reducing accidental unplugging and improving contact pressure HowStuffWorks. -
Locking & Safety Accessories
- Extension-cord locks – DeWalt’s Lighted Locking CGM cords twist a collar that cams into the holes, holding up to 40 lb of pull force and preventing dangerous arcing if a cord is yanked DEWALT.
- Plug locks & barricades – Job-site devices like PlugLocker use a clamshell latch that bites into the holes to keep power tools energized without tape Amazon.
- Child-safety caps – Some tamper-resistant caps hook through the holes so curious fingers can’t pry them out easily.
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Low-Cost Theft Deterrent
Contractors often run temporary power outdoors. Locking cords that exploit blade holes deter theft of tool cords—nobody can unplug without twisting the proprietary collar first.
Choosing the Right Locking Solution
| Use Case | Recommended Product | Key Feature |
| Heavy-duty tools & compressors | DeWalt Lighted Locking 12/3 SJTW | Twist-to-lock, neon power-on light |
| General job-site safety | PlugLocker replacement ends | Retrofit any cord with 40 lb retention |
| Home & hobby | Locking outdoor cord with sliding collar | Weather-resistant gasket, easier one-hand release |
Tip: Look for cords that also carry an internal neon or LED lamp—it uses the ground wire as a return path, showing at a glance that the circuit is live.