The perception of music is based on our capacity to identify musical sounds by their relationship to a given key (or root) - not immediately by their absolute pitch. (The sense of perfect pitch is a faculty that is in most cases innate and independent of the direct perception of music.) The advantage of relative solmization over absolute tone names is that it also expresses the musical 'meaning' (function) of the notes.

According to Kodály’s musical concept, each major scale has a

      By raising the DO, we obtain degree DI,
      by raising the FA, degree FI,
      by raising the SO, degree SI, etc.
      By lowering the MI, we obtain degree MA,
      by lowering the TI, degree TA, etc.

For example, in the C major (or A minor) key:

      LA-DI-MI stands for the A major triad (A-C^#-E),
      DO-MA-SO stands for the C minor triad (C-E^b-G).

The chromatic scale in the E major or C^# minor key runs as shown in the next figure:

Relative solmization is more than just a matter of learning how to write and read music. It is virtually the only method which is capable of expressing modal-polymodal relations. In it, for example,  the closedness of the system is expressed by the fact that DO-DI and MI-MA are not only derivatives but also mirror images of each other. This method can tell us something profoundly new about the modal structure and semantics of music.