The Kalin Twins (also known as Hal and Herbie both born February 16, 1932) were an American pop singing, songwriting and recording duo, formed in 1958 by twin brothers Harold Kalin and Herbert Kalin. The duo is best remembered for their number one 1958 hit “When”.

The twins were born in Port Jervis, New York, United States, but the family later moved to Washington, D.C. Originally discovered by Clint Ballard, Jr., the writer of many hit records such as “Good Timin'” for Jimmy Jones, and “I’m Alive” for The Hollies, the sibling duo had a couple of early recording flops.[6] However, in 1958, after searching through piles of writers' demo tapes, their management discovered the song called “When”, written by Paul Evans and Jack Reardon.
It topped the UK Singles Chart on 13 September 1958, got to Number 5 in their U.S. homeland, and sold over two million copies in the process. The track remained in the UK listings for eighteen weeks, five of which were at Number One. They had no further UK chart entries.

The Kalins were the first set of twins to reach number one in the UK as a duo, followed years later by The Proclaimers. They were supported by Cliff Richard on their only UK tour. Their second single, “Forget Me Not,” reached Number 12 in the US Billboard chart later in 1958. After two further low-ranking entries in 1959, they never reached the charts again.