Is it safe to insert a 3-prong plug into a 4-prong socket?

Since 3- and 4-prong outlets are installed to accept cords that are wired differently, it is not recommended that 3-prong plugs be plugged into 4-prong outlets. Instead, it is recommended that 3-prong cords be converted to 4-prong cords via an adapter.

Cords designed for 3-prong outlets have three wires. Likewise, cords designed for 4-prong outlets have four wires. Therefore, it is very likely that 3-prong appliances will not work properly, if at all, when plugged into 4-prong outlets. Cords can be purchased to convert a 3-prong cord to 4-prong cord, though it is not currently legal to convert a 4-prong cord to a 3-prong cord, since 4-prong cords are the most recent standard.