Tonality refers to the harmonic language used in the music. This is about the way notes are chosen and how they’re combined. Tonality is a complex topic, but a good way to approach it is to look at two ways to express tonality – major/minor and consonant/ dissonant.
(The following explanations are deliberately simplistic – intended only as a quick introduction, not a comprehensive discussion of music theory.)
Major tonality is most strongly expressed as the major-third interval from the tonic. For example, if your song is in the key of C, the major-third from C is the note E-natural (white note, with no sharps of flats). So, using a lot of E-natural notes will give your song a strong major feel. If your song is in a different key, the note relationships remain the same. So, if your song’s tonic is F#, the major-third will be the note A#. While the major third is the strongest way to express a major tonality, major-sixth and major-seventh from the tonic also contribute to a major tonality.
Similarly, a minor tonality is most strongly expressed as the minor-third interval from the tonic. For example, if your song is in the key of G minor, the minor-third from G is B-flat. So, using a lot of B-flats in your song will give you a strong minor feel.
Exclusive use of major or minor tonalities can create too stark an effect – like using too much of a single colour. Often, it makes sense to combine major and minor tonalities in varying degrees throughout a song. A more balanced sound can be achieved by using some major chords and some minor chords – even having some song sections predominantly major and other song sections predominantly minor.
Consonant tonality sounds like the harmonic and melodic content is clear and unambiguous. An extreme form of consonance is a musical part where all pitched instruments are playing the same note. Octaves, fifths, fourths and thirds are all quite consonant.
Unlike consonance, dissonant tonality usually sounds crowded and ambiguous. This is usually caused by harmonic combinations that are complex and even clashing. Minor seconds, tritones, and major sevenths can often be combined to create dissonant tonalities.
Like major and minor tonalities, exclusive use of consonance or dissonance can sound too stark. Having some sections that are more consonant and other sections more dissonant is a great way to give your song a subtle sense of ebb and flow.