Next in importance to harmony as a mode of unity in variety in architecture is balance. Balance implies emphatic variety, or contrast. One mode of balance, that between the upward and the downward tendencies, we have already discussed. There is another mode, similar to that which exists in painting and sculpture, the balance between the right and left members of a building. In order that this type of balance may be appreciated, there must be some axis or line of mediation between the parts, from which the opposing tendencies take their start; otherwise we view the parts together, instead of in opposition. For example, there is balance between two wings of a building which are separated by some central member or link; balance between the aisles of a church on either side of the nave; balance between the sets of three columns right and left of the door in the Greek hexastyle temple. Such cases of symmetry between equal right and left parts are the simplest examples of balance; but there are other, more complex types. For example, the parts may be unequal, yet balance nevertheless, provided their inequality is compensated for by some enrichment of design or ornament in the lesser part. Or again, there may be a balance between contrasting shapes, such as the square and the triangle, when they make an equal claim upon the attention. Although, since architecture is a static art, evolution is not so important as harmony and balance, it exists nevertheless. In a colonnade, as you look down it, with the height of the columns diminishing in perspective, there is a rhythmical movement of eye and attention toward the last column as a goal. There is the same rhythmical movement in following the arches on either side of the nave of a church leading to the apse. There is a rhythmical movement in the progressive diminution of the height of the stories of a building, going towards the top. In such spatio-temporal rhythms, the proportional equality between the members corresponds to the equal intervals in temporal rhythms, and the alternation between member and intervening space, or between member and line of division, corresponds to the alternation between heavy and light accents. Last, evolution is present in architecture, whenever, often without rhythmical divisions, the attention is impelled to move along lines that meet at a point which serves as a climax, as in all triangular forms where the lines lead up to the apex,--pointed windows or arches, towers ending in belfries or pinnacles.