Typography and Page Layout: Typesetting
http://www.typography-1st.com/typo/typset.htm

Magnik, John
Typography First

Abstract:

Typing or setting text lines to the same length so that they line up on the left and the right is known as 'justification.' The information that you are now reading has been typeset using this method. 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.