How to know if you need a plug adapter (or a voltage converter)
These are two different questions, and most articles online muddle them together. Here's the clean version:
Plug adapter = shape
Your plug fits the wall socket, or it doesn't. Countries use different physical shapes — Types A through O. If your country's plug shape matches your destination, you don't need a plug adapter. If it doesn't, you do, full stop.
Voltage converter = electricity
The wall socket delivers either low voltage (around 100–127V; USA, Canada, Mexico, Japan) or high voltage (around 220–240V; most of Europe, UK, Asia, Africa). Modern dual-voltage devices (phones, laptops, most chargers) handle both. Look on the brick for a line that says something like INPUT: 100–240V — if you see that, you're fine with just a plug adapter.
Single-voltage devices — hair dryers, curling irons, flat irons, travel kettles, anything that heats at high wattage — usually don't. A cheap adapter plus a single-voltage device plugged into the wrong country will pop a breaker or melt the device. If you have to travel with one, buy the dual-voltage travel version.
Universal sockets (the footnote)
Some countries — Bangladesh, Cambodia, Ghana, Maldives, Myanmar, Nepal, Nigeria, Pakistan, Yemen, and others — have socket designs that physically accept multiple plug types. In those places a traveller's plug often fits without an adapter even when the national "official" plug type is different. Our checker flags these with a "probably not needed" verdict so you know what to expect.