From TalkBass Wiki
In 1976 D'Addario developed the Half Round grinding process, which made available the "third kind of string." Each Half Round string starts as an oversized precision round wound string and is centerless ground to a smooth, precise final diameter. With a tone between their round wound and flat wound peers, D'Addario Half Rounds offer the advantages of a smooth exterior string (reducing finger noise and fret wear) while maintaining some of the brightness of a round wound string.
