First Rectilinear Composition
The dominant form in my first composition is the large horizontal block, since it acts as the main form that the others are attached to. The subdominant form is the thin vertical piece coming out of the top of the dominant block. The subordinate piece is the small slim form intersecting the dominant form.
Second Rectilinear Composition
The dominant form in this composition is the large block at the edge of the composition. The thin piece cradled by the dominant piece is the subdominant form. And the small vertical piece wedged into the subdominant piece is the subordinate. The thin gap between the subordinate and the dominant creates interesting negative space along the vertical axis.
Third Rectilinear Composition
The dominant form in this composition is of course the thick block in the center. The subdominant is the vertical form, and the subordinate is the thin piece intersecting both the dominant and subdominant. The subordinate piece creates interest by incorporating all three forms into one.