Some of the most important systems for specifying the octave of a note are shown below:

1. Scientific system.
2. MIDI (Musical Instrument Digital Interface) system. 60 = middle C, 61 would be C# or Db, 62 = D, etc.
3. Helmholtz. Developed by Hermann von Helmholtz in 1863.
4. Italian system.
5. English system.

---

---