The following extract is from "On Painting", by Leon Battista Alberti, and first describes the use of drawing grids:
"Here is that which will be of great use to he who desires to use it. I think that there is nothing more useful than this 'veil' which, when I am with my friends, I call a grid.
Leon Battista Albert
First, it always presents you with the plane, unaltered. There where you have marked precises limits, you will easily find the pyramid's true cuspid, which would assuredly be difficult without the grid. You are aware of how impossible it is to copy something that does not continually have the same appearance, for it is easier to imitate painting than sculpture. You area aware that as the centre's distance and position of the centre are modified, the thing you see also appears to change greatly.Therefore, as I said, the veil will be, quite useful to you, since that which you see is always the same thing.
Second, it will be easy for you to find where the outline and the planes lie. Here on this parallel line you will find the forehead, on that the nose, on another the cheeks, on this lower one the chin and all the outstanding features each in their place. You will be able to put everything in its right place, on walls or panels divided up with similar parallel lines.
Last, the veil will help you greatly in learning to paint when you see round objects and objects in relief through it. It is in this manner that you will be able to use your experience and judgement to see how very useful our veil can be to you.
Also, I will not listen to what some may say, that the painter should not use these things, because even though they are a great help in painting well, perhaps they are such that he will soon be unable to do anything without them.
I do not believe that a painter should be expected to suffer endless pain, but rather to make paintings with good relief and that are a good likenesse of their subject. I do not believe this can ever be done without using the veil. Therefore, let us use this grid, or veil, as we have said. Then, when a painter wishes to test his skill without the veil, let him first mark the edges of the objects within the veil's parallel lines, or he may study them differently in imagining a line intersected by its perpendicular wherever these limits fall."
- Leon Battista Alberti