To choose a width of column which makes the text pleasant to read is one of the most important typographic problems. The width of the column must be proportioned to the size of the type.
Overlong columns are wearying to the eye and also have an adverse psychological effect. Overshort columns can also be disturbing because they interrupt the flow of reading and put the reader off by obliging the eye to change lines too rapidly. Lines which are too short or too long reduce the memorability of what is read because too much energy has to be expended. There is a rule which states that a column is easy to read if it is wide enough to accommodate an average of 10 words per line. If the text is of any length, this rule is of practical help. A small amount of text can be set in long or very short lines without disturbing the reader.
Sufficient leading between the lines is of the first importance for easy reading. If the lines are too closely set the eye is forced to “take in” the neighboring lines while reading. Anything that might impair the rhythm of reading should be scrupulously avoided.