This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
In drawing in pen and ink, all effects of shadow are made by lines, and different values are obtained by varying the width of lines, or of the spaces between the lines, or by both. In any case the integrity of each line must be preserved and there can be very little crossing or touching of shade lines, as that causes a black spot in the tone unless lines cross each other systematically and produce cross hatching. With the pencil, however, owing to its granular character one may produce a tone without any lines; a tone made up of lines which by touching or overlapping produce a soft, blended effect, in which the general direction of the strokes is still visible, or a tone made up of pure lines as in pen work. In general it does not matter so much, as in pen drawing, if lines touch or overlap. Indeed, the natural character of the pencil line leads to a treatment which includes both pure lines and more or less blended effects.