Tuner apps are mobile applications designed to help musicians and music enthusiasts tune their instruments accurately. They use the device's microphone to detect the pitch of the sound produced by an instrument, such as a guitar or violin, and provide visual feedback to indicate whether the pitch is too high, too low, or in tune. These apps often feature various tuning modes, allowing users to select different reference pitches or tunings, such as standard tuning or drop D tuning. Many tuner apps also include additional tools, like metronomes, to assist with practice and performance.