According to Sibelius, Finnish folk-tunes are based on "simply five tones d e f g a to which are added two more tones, b and c, when the melody takes on a climactic character". Sibelius's modes can be described as formed of a pentachord and a tetrachord, with the first degree of the pentachord as final or tonic. Modes based on different finals but sharing the same collection of pitches may be called scalarly associated. 'Mixed modes' may be explained as combinations of the lower pentachord of one mode with the upper tetrachord of another mode. The paper describes the use of these modes and of their characteristic chords in Sibelius's compositions.