 The Unix typesetter program troff uses two spaces to mark the end of a sentence . This allows the typesetter to distinguish sentence endings from abbreviations and to typeset them differently . Early versions of troff , which only typeset in fixed width fonts , would automatically add a second space between sentences , which were detected based on the combination of terminal punctuation and a line feed . 
 The Unix typesetter program troff <a href='uninflect' title='1'><mark>uses</mark></a> two spaces to mark the end of a sentence . This <a href='uninflect' title='2'><mark>allows</mark></a> the typesetter to distinguish sentence endings from abbreviations and to typeset them differently . Early versions of troff , which only typeset in fixed width fonts , would automatically add a second space between sentences , which <a href='uninflect' title='3'><mark>were</mark></a> detected based on the combination of terminal punctuation and a line feed . 
Da Unix typesetter program troff use two spaces to mark the end of a sentence . dis allow the typesetter to distinguish sentence endings from abbreviations and to typeset em differently . Early versions of troff , which only typeset in fixed width fonts , would automatically add a second space between sentences , which was detected based on tdacombination of terminal punctuation and a line feed .