The W stands for weather-resistant. That's it.

If the cord has a W, it’s rated for outdoor use. Without it, it’s indoor only.

SJTW is a common cord rating. The S is “Service” (600V), J is “Junior” (300V), T is “Thermoplastic” insulation. The W means the jacket can handle sunlight, moisture, and temperature swings. Without the W, the insulation can crack and expose wires.

So when you’re buying an extension cord for outside — string lights, power tools, holiday decorations — make sure you see that W. A plain SJT cord is fine in the garage, but don’t leave it in the rain.

If the cord doesn’t have a W, bring it inside. Future You deserves a working circuit.