The island is one of a group of islands called the Grenadines, most of which are part of St Vincent and the Grenadines, in the West Indies.
Most U.S. airports use the same three-letter location identifier for the FAA and IATA, but Chester County Airport is MQS to the FAA and CTH to the IATA (which assigned MQS to Mustique Airport on Mustique island in Saint Vincent and the Grenadines.)