This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
The following remarks will be found to contain, in a condensed form, the rules laid down by Messrs Glenny, Turner, and other prominent exhibitors of the Dahlia: First, then, we recommend young exhibitors to cut the flowers that are most perfect; that is to say, with centres as full up and outlines as fine as they can find them, without regard to colour or size. Of course, among these there will be some large, some middling, and some even less than middling, therefore the next thing is to divide them into the three sizes. Beginning, then, with the largest that are perfect, they must begin with the back row. If they can, they should put the best two light ones at the top corners, and the best two dark ones next to them, working towards the centre. The next two must be the next best light ones they can produce, and the next best dark ones in the centre. They may then look to the middle-row size, and put the best two dark ones at the two ends, under the two top corners; two light ones next under the top dark ones, then two other dark and two light ones in the centre.
The bottom row is done after the plan of the top; and when these are all arranged, look among the spare flowers to see if any are better than you have already placed, but be careful that you have no duplicates, because two of one sort disqualifies the stand. It is just possible that you may be deficient of light or dark flowers, and unable to carry out the plan to your liking. In this case, see if you can mend matters by putting larger flowers in the second or third row, instead of carrying out the three sizes complete, remembering the four corners are always the more striking when light. Again, it may be that light flowers are scarce; you must then substitute the brightest. Now whites, edged flowers, lilacs, and yellows, and even orange colours, may be fairly used as light. Next to these, bright scarlet may be so appropriated; but purples are all dark, and heavy crimsons; and even when a majority of dark flowers prevents us from doing as we wish, still a great deal may be done by uniformity of arrangement, not to have two dark ones together, except in the centre, nor two light ones. When they all run too much of one character, there will be shades of difference to enable us to do something towards uniformity.
A stand of twelve must be done in the same way, keeping the outsides in the top and bottom rows as light as we can, and the centres as dark as we can, and the middle row reversed. We are quite sure that uniformity in a stand is a strong point in its favour; and judges, if they do their duty, are bound to notice it so far that when two stands are equal in other respects, the stand properly arranged should have the benefit. Simple as this may seem, let any one go over a number of stands in an extensive show, and he will see more than half of them set up without the slightest attempt at arrangement, and many a stand of really good flowers spoiled by a total disregard to any kind of contrast - one end of the stand, perhaps, with nearly all dark and the other light, or otherwise the effect spoiled by a bad disposition of the flowers.