In the U.S. , two-blade plugs usually have one blade larger than the other.
Outlets are the same way.
The larger blade is the neutral side of the current. This is a safety feature intended so the plug can be inserted one way only to reduce the chance of accidental shock. If you try to plug a modern plug into an old-style receptacle for equal size blades, it won't go in unless you file down the larger blade to the older plug size. Outside the US, many countries with Type A use the old style plug, and a newer US plug with unequal pins might pose a problem. Also found in Azores, Belize, Trinidad and Tobago.