Each letter stands for an aspect included in the cord: S - service OO - oil resistant insulation and oil-resistant jacket W - weather resistance This means an SOOW cord consists of an oil-resistant insulation and jacket as well as weather resistance. Other types of cords may use other letters to indicate other aspects, such as: J – junior cable (rated to 300V instead of 600V) T – thermoplastic E – thermoplastic elastomer O – oil-resistant outer jacket