This master thesis is an artistic exploration into the meaning of the word modal, when describing a certain approach to improvisation and composition in a jazz context, and the attempt to pursue and develop such an approach. The purpose of this work was to gain new knowledge within the areas of melodic and harmonic expression, by means of drawing upon personal musical strengths and ways of thinking about music. Suggestions on how to define a modal approach and what to associate it with, lead to artistic experimentation, which was an important part of both the early and late stages of the process. Such experimentation in regard to mapping out shifts between different modalities, resulted in the creation of a harmonic structure referred to as the Yin and Yang system. This system was then used as a tool for composing and arranging music for jazz quartet and quintet settings. Harmonic progression generated by the system possesses a characteristic sound and invites non-functional melodic improvisation, which were the traits that the repertoire of new music was built around. Work with the Yin and Yang system and the music connected to it, amounted to the largest part of this thesis.