Fork or cross fingerings consist in opening a hole, and closing one or two holes below it, as shown in the following examples. This generally has the effect of lowering the note by a semitone
Fork fingerings have a different, more sombre tone than simple fingerings, because their partials are not in tune with each other. For this reason each key has its particular character, according to their number in the scale. This contributes to the charm of the recorder (and of other early wind instruments that work on the same principle).
partials are the frequencies that compose the sound spectrum of a musical note and are responsible for its tone colour.
They are called harmonics when they are in tune with the fundamental note.