The derivative r'(θ) is proportional to the parameter "b". In other words, it controls how "tightly" and in which direction the spiral spirals. In the extreme case that "b" = 0 ("ɸ" = π/2) the spiral becomes a circle of radius "a". Conversely, in the limit that "b" approaches infinity ("ɸ" → 0) the spiral tends toward a straight line. The complement of ɸ is called the "pitch".