The term 'Either of Two 'UNFORGETTABLE' Singers' refers to two iconic singers from the 20th century, Nat King Cole and Natalie Cole. Nat King Cole was an American jazz pianist and vocalist who had a successful career in the 1940s and 1950s. His daughter, Natalie Cole, was also a successful singer, known for her soulful voice and her Grammy-winning cover of her father's song 'UNFORGETTABLE'. Both Nat King Cole and Natalie Cole are considered two of the most influential singers of the 20th century.