Some of the most important systems for specifying the octave of a note are shown below:
- Helmholtz. Developed by Hermann von Helmholtz in 1863.
- English system.
- Italian system. It uses note names do-re-mi-fa-sol-la-si. The French system is the same but it uses ut instead of do.
- Scientific system.
- MIDI (Musical Instrument Digital Interface) system. 60 = middle C, 61 would be C# or Db, 62 = D, etc.