IPA: /ˌdaɪəˈtɒnɪkli/
KK: /daɪəˈtɑnɪkli/
In a way that relates to the diatonic scale, which consists of seven pitches in a specific pattern of whole and half steps.
The melody was composed diatonically, following the natural scale without any chromatic alterations.
Diatonically → It is formed from "dia-" (meaning through or across) and "tonos" (from Greek, meaning tone). The word refers to a musical scale that progresses through tones, emphasizing the natural sequence of notes.
Think of 'through' ('dia-') and 'tone' ('tonos') to remember that 'diatonically' relates to music that moves through a series of tones.