Type B
Direct Answer
Type B adds a round grounding pin to Type A's two flat blades -- NEMA 5-15, the most common outlet in North America. The ground pin is deliberately longer than the power blades, so it connects before and disconnects after them, a safety-sequencing feature shared with several other grounded plug types worldwide.
Source: NEMA 5-15
Specs
| Pins | 2 flat blades + 1 round ground pin (longer than the blades) |
| Rated Current | 15 A |
| Rated Voltage | 125 V |
| Grounded | Yes |
| Common Regions | USA, Canada, Mexico, Central America, Japan (grounded appliances) |
Ground First, Ground Last
NEMA 5-15 is described as the single most common electrical outlet in North America. Its defining feature beyond the two flat power blades is a round ground pin positioned so it's physically longer than the blades -- meaning it makes contact in the socket before the live and neutral blades do when plugging in, and stays connected longer as the plug is pulled out. That sequencing is a deliberate safety feature: the appliance's chassis is grounded before it can receive power, and stays grounded until after power is cut.
Type B sockets also accept ungrounded Type A plugs without issue, since Type A's blade dimensions are identical -- the ground pin hole is simply left empty.