When Octaves Flat is open and your iPhone is exposed to sound, the display shows spots corresponding to distinct pitches, or notes. The horizontal axis is labeled with the chromatic scale, the notes C through B. The vertical axis is labeled with standard octave numbers. A4 is aligned with modern standard concert pitch, 440 Hz.
Portrait and landscape modes are both supported, as you prefer. The labels and guide lines can be hidden by tapping anywhere on the screen, and shown again the same way. Additional options are found by swiping left anywhere on the screen.
A musical tone usually consists of more than one pure tone. The fundamental tone is the lowest pure tone in a musical tone. A clear whistle is nearly a pure tone, and will appear in Octaves Flat as a single spot. The human voice has a strong fundamental tone and usually several significant overtones, appearing as additional spots above the fundamental in Octaves Flat, depending on the voice. Some musical instruments have overtones nearly as strong as the fundamental tone, but the fundamental tone always has the lowest pitch, and appears vertically lowest on the display in Octaves Flat.
To identify the tuning of an instrument, the fundamental tone is considered and the overtones ignored. When using Octaves Flat for tuning, you are looking for the largest, lowest spot to indicate the note.
Beware background noise! During development I discovered that some household appliances generate a quite loud electrical hum that I had learned to ignore. In my home, a 60Hz hum is often present, and appears as a note between A1♯ and B1.
For help using Octaves Flat, to report an issue, or to request a feature, send an email here.