Which instrument is commonly used to test a singer's vocal range?

The piano is commonly used to test a singer's vocal range because it provides a clear and precise reference pitch that allows the singer to match their voice to the notes played. The keyboard layout makes it easy to identify specific notes and facilitates the testing of different ranges by providing both melody and harmony. Singers can sing various scales, arpeggios, or specific songs using the piano to assess how high or low they can sing while maintaining tonal quality and control. Additionally, the comforting familiarity of the piano in vocal training makes it an ideal choice for this purpose, as it helps singers focus on their vocal capabilities without the additional complexity that might come from other instruments.