Quinterval is a free iPhone application developed by Oliver Hahn that offers an interactive circle of fifths with a range of features for music enthusiasts. This multimedia program is categorized under Music & Radio and provides a comprehensive display of corresponding scales.

The main feature of Quinterval is its interactive circle of fifths, which allows users to select a key by either touching the tonic or using the slider in the middle. The circle displays both the major and parallel minor scales for the selected key. The visible section of the circle can be adjusted using a second slider, providing a customizable and dynamic experience.

Quinterval also offers options for note names, allowing users to choose between German, English, or Italian notation. The lower area of the app displays the related major scale with its key signature, highlighting the first and sixth steps along with the corresponding minor scale. Users can also choose to display the notes with G-Clef, C-Clef, or F-Clef, and can opt to display accidentals before the individual notes.

For those seeking additional functionality, Quinterval offers in-app purchases such as the ability to display the notes of the parallel minor scale and display triads with their chord names. Users can also tap on the notes to play them back, adding an interactive and engaging element to the learning experience.

Quinterval is a valuable tool for musicians, music students, and anyone interested in exploring the circle of fifths and scales. Its intuitive interface, customizable options, and additional features make it a must-have for music enthusiasts on the iPhone platform.