Alternate

Alternate Gradation - Alternate gradation provides unusual complexity in a gradation design. It means that gradually changing unit forms or structural subdivisions from opposite directions are interwoven together. The simplest way to achieve alternate gradation is to divide the structure [either the vertical or the horizontal rows] into odd and even rows, and have all the odd rows observe a discipline different from the even rows..... Manipulating the range, speed, and direction of gradation, we can have almost unlimited kinds of variation.....

If the unit forms are in gradation of size, the space left over by diminishing unit forms can be used for the accommodation of a set of unit forms in reverse gradation. [For example] the original unit forms can occupy the central portion of the structural subdivisions, whereas a new set of unit forms can occupy intersections of the structural lines.

In a gradation structure, alternate gradation can be obtained if the A rows gradually diminish while the B rows gradually expand simultaneously in the same direction. ...the combined width of every pair of A and B rows should always remain constant [or in very slow gradation]....[Wong, Wucius. Principals of Two-Dimensional Design. New York: Van Nostrand Reinhold Company, 1972.]

From Ruskin - "The Law of Interchange" [Alternation as Interchange-Interchange/receiving something imparted]

Included in the text that follows:
Interchange enforces the unity of opposite things, by giving to each a portion of the character of the other

Sometimes this alternation is merely a reversal of contrasts

The appearance of intentional artifice with which contrasts of this alternate kind are produced [in nature]

Light and shadow are respectively interchanged by way of contrast, but in similar proportions.....

Opposite natures may be helped and strengthened by receiving each, as far as they can, some impress or reflection, or imparted power, from the other.

Text
Closely related with the law of contrast is a law which ^enforces the unity of opposite things, by giving to each a portion of the character of the other. If, for instance, you divide a shield into two masses of colour, all the way down--suppose blue and white, and put a bar, or figure of an animal, partly on one division, partly on the other, you will find it pleasant to the eye if you make the part of the animal blue which comes upon the white half, and white which comes upon the blue half. This is done in heraldry, partly for the sake of perfect intelligibility, but yet more for the sake of delight in interchange of colour, since, in all ornamentation whatever, the practice is continual, in the ages of good design.

Sometimes this alternation is merely a reversal of contrasts; as that, after red has been for some time on one side, and blue on the other, red shall pass to blue's side and blue to red's. This kind of alternation takes place simply in four-quartered shields; in more subtle pieces of treatment, a little bit only of each colour is carried into the other, and they are as it were dovetailed together. One of the most curious facts which will impress itself upon you, when you have drawn some time carefully from Nature in light and shade, is the appearance of intentional artifice with which contrasts of this alternate kind are produced by her; the artistry with which she will darken a tree trunk as long as it comes against light sky, and throw sunlight on it precisely at the spot where it comes against a dark hill, and similarly treat all her masses of shade and colour, is so great, that if you only follow her closely, every one who looks at your drawing with attention will think that you have been inventing the most artificially and unnaturally delightful interchanges of shadow that could possibly be devised by human wit.

You will find this law of interchange insisted upon at length by Prout in his lessons on Light and Shade: it seems of all his principles of composition to be the one he is most conscious of; many others he obeys by instinct, but this he formally accepts and forcibly declares ["...^light and shadow are respectively interchanged by way of contrast, but in similar proportions....."]

The typical purpose of the law of interchange is, of course, to teach us how opposite natures may be helped and strengthened by receiving each, as far as they can, some impress or reflection, or imparted power, from the other.

[On Composition, pgs. 196-198, The Elements of Drawing, John Ruskin, Dover Publications, Inc., New York, 1971 [Originally Published in London, 1857]

R  E  F  E  R  E  N  C  E  S 

2 Alternate vt [1599] 1: to perform by turns or in succession 2: to cause to alternate -vi: to change from one to another repeatedly [rain alternated with sun]

3 Alternate n [1717] 1: aAlternative 2: one that substitutes for or alternates with another

Alternation n [15c] 1a: the act or process of alternating or causing to alternate b: alternating occurrence: Succession 2: Inclusive Disjunction 3: the occurrence of different allomorphs or allophones