An app that imitates a tuning fork. You can change the frequency with the displayed buttons. Plus and minus will bring you one Hertz lower or higher. The arrows will bring you halftones up and down or to other useful frequencies in between.

That way the app is not only useful for orchestra musicians and classical or pop singers, but also for musicians and singers in early, baroque, renaissance, romantic and classical music. Just change the frequency to 430, 415, 392, 466 or other historical chamber pitches fast and easy.

You can also set a default frequency if you tend to use a specific one very regularly, so you don't have to change it everytime you open the app.