Sentence spacing concerns how spaces are inserted between sentences in typeset text and is a matter of typographical convention. Since the introduction of movable-type printing in Europe, various sentence spacing conventions have been used in languages with a Latin alphabet. These include a normal word space (as between the words in a sentence), a single enlarged space, and two full spaces.
Until the 20th century, publishing houses and printers in many countries used additional space between sentences. There were exceptions to this traditional spacing method—some printers used spacing between sentences that was no wider than word spacing. This was French spacing—a term synonymous with single-space sentence spacing until the late 20th century. With the introduction of the typewriter in the late 19th century, typists used two spaces between sentences to mimic the style used by traditional typesetters. While wide sentence spacing was phased out in the printing industry in the mid-20th century, the practice continued on typewriters and later on computers. Perhaps because of this, many modern sources now incorrectly claim that wide spacing was created for the typewriter.
The desired or correct sentence spacing is often debated, but most sources now state that an additional space is not necessary or desirable. From around 1950, single sentence spacing became standard in books, magazines, and newspapers, and the majority of style guides that use a Latin-derived alphabet as a language base now prescribe or recommend the use of a single space after the concluding punctuation of a sentence. However, some sources still state that additional spacing is correct or acceptable. Some people preferred double sentence spacing because that was how they were taught to type. The few direct studies conducted since 2002 have produced inconclusive results as to which convention is more readable.
History
Traditional typesetting
Shortly after the invention of movable type, highly variable spacing was created, which could create spaces of any size and allowed for perfectly even justification. Early American, English, and other European typesetters' style guides (also known as printers' rules) specified spacing standards that were all essentially identical from the 18th century onwards. These guides—e.g., Jacobi in the UK (1890) and MacKellar, Harpel, and De Vinne (1866–1901) in the U.S.—indicated that sentences should be em-spaced, and that words should be 1/3 or 1/2 em-spaced. The relative size of the sentence spacing would vary depending on the size of the word spaces and the justification needs. For most countries, this remained the standard for published work until the 20th century. Yet, even in this period, there were publishing houses that used a standard word space between sentences.
Mechanical type and the advent of the typewriter
Mechanical type systems introduced near the end of the 19th century, such as the Linotype and Monotype machines, allowed for some variable sentence spacing similar to hand composition. Just as these machines revolutionized the mass production of text, the advent of the typewriter around the same time revolutionized the creation of personal and business documents. But the typewriters' mechanical limitations did not allow variable spacing—typists could only choose the number of times they pressed the space bar. Typists in some English-speaking countries initially learned to insert three spaces between sentences to approximate the wider sentence spacing used in traditional printing, but later settled on two spaces, a practice that continued throughout the 20th century. This became known as English spacing and marked a divergence from French typists, who continued to use French spacing.
Transition to single spacing
In the early 20th century, some printers began using one and a half interword spaces (an "en quad") to separate sentences. This standard continued in use, to some extent, into the 1990s.
Magazines, newspapers, and books began to adopt the single-space convention in the United States in the 1940s and in the United Kingdom in the 1950s. Typists did not move to single spacing simultaneously.