Why electrical plug prongs have holes in them
Engadget explains that the holes in plug blades once helped outlets grip the plug, but today they mostly help factories hold parts in place.
The facts
- According to Engadget, the holes mainly help manufacturers position the metal blades while plastic is moulded around them, using an alignment rod.
- Citing the National Electrical Manufacturers Association (NEMA), the holes originally worked with a small raised "detent" inside the outlet that clicked into place to hold the plug.
- Underwriters Laboratories (UL) later revised its receptacle standards so outlets hold plugs without the holes, and most manufacturers dropped the matching raised feature.
- Engadget says a plug with solid blades is not defective, and a loose plug is more likely a worn outlet than a problem with the prongs.
Case notes · Danny's take
Why it matters
A feature can outlive its reason for existing. The holes once did a job in your wall. Now they do a job in the factory, and nobody notices. Plenty of tech works like this, with old design choices kept because changing them costs more than leaving them. Before you trust a popular explanation, ask where it came from.
For builders
Compatibility rules can keep a design feature alive long after its original purpose is gone. If you make hardware, check which parts of your design are there for the user and which are there for your production line.
Not financial advice. Do your own investigation.
This story was summarised with AI from Engadget's reporting and checked against our editorial policy. Danny's take reflects his views. Disclaimer.
Danny Walter
Founder of Moon Republic, Volt Terminal and Moon Republic Academy. Author of The Market Is a Crime Scene and After the Applause.