The Kalin Twins were an American pop music duo, comprising identical twin brothers Hal and Herbie Kalin. Known for their distinctive close harmony style, their most notable hit was "When," which reached number 1 on the UK Singles Chart and number 5 on the Billboard Hot 100 in 1958. Despite their early success, they are often categorized as one-hit wonders due to their lack of subsequent chart-topping songs. Their influence in pop music history is largely tied to their unique position as identical twin performers in a genre dominated by solo artists and non-related groups.