I Ching - The Lines
The nature of the line's force is such that it is continually transforming over time, from yin to yang and back again in a continuous cycle of change.
Thus it is that the combination of lines within a hexagram provides a snapshot of a situation, whose character is not only complex but also evolving.
Furthermore, the concept of the hexagram dictates that in isolation a line is meaningless. It is only when taken within the context of the other lines, and the hexagram as a whole, that it becomes significant.
In Book III of his translation, Wilhelm elaborates on the relationships between the lines. In Book I - the part reproduced on this site - the emphasis is upon interpreting the meaning of the lines in relation to the hexagram itself.