The practice originated with Mediaeval scribes who ruled margins and text lines so as to speed writing and fit as many characters on a line as possible. The opposite of justified text is ragged text. Justification is accomplished by filling a line until the last possible word or syllable fits and then dividing the remaining space by the number of word spaces. The result is placed at each word space. Word spaces are variable in width, expanding or contracting as needed to space the line out to its justification width.
Margins are the imaginary vertical demarcations for text or tabular columns.
Space between lines To improve the ease with which text can be read, i.e., legibility, the amount of extra space placed between each line in a setting must be varied according to the line length and character of the type face being used. A suggestion in this matter is to have space equal to the depth of the lower case 't' between the lines. Most settings will always benefit with the addition of extra line space. Generally, the longer the line, the more space that is required. This is certainly the case when deciding the line spacing of sans serif type faces and for paragraphs set in a bold face.