The NEF was a series of escort groups usually composed of up to four ships with two to three corvettes and one destroyer. The groups were given numeric designations between 14 and 25. As part of its creation, the United Kingdom agreed to return Canadian destroyers operating in European waters. Also augmenting Canadian vessels were ten Royal Navy corvettes that were handed over to the Royal Canadian Navy for use in the NEF, and additional Royal Navy s. The senior officer in the escort group automatically became the Senior Officer Escort and took charge of convoy defence along with commanding his own ship.