When the typewriter was introduced in the late 1800s, typists utilized two spaces between sentences to emulate the manner of conventional typesetters. While wide sentence spacing was phased out in the publishing business in the mid-twentieth century, it persisted on typewriters and, subsequently, computers. Today, one space is used instead.
The Evolution of Two Spaces Before word processors, two spaces after a period were necessary and taught as right if you learnt to type on a typewriter. Because the spacing between words on a typewriter was irregular, the extra space was required to define the beginning of a new phrase. Today, most word processors allow you to choose how many spaces to use before and after a period. Some recommend one space, others two. There is no correct answer as it depends on your personal style.
He argues that the practice of two spaces between sentences dates back to the days when everyone wrote on manual typewriters. Because of how typewriters handled proportional spacing, typewritten manuscripts with two periods between sentences were simpler to read. This argument is not without merit; during a time when computers were not widely used, good typists could make more money than good programmers.
The two-space rule has been criticized for being inconsistent and preventing proper indentation of long sentences. However, it still is widely accepted as such by most law firms and their clients.
There are several reasons why lawyers put two spaces after a period. First, it is an inherited custom. The late John Henry Wigmore, who was a leading authority on evidence law, argued that the two-space rule was derived from the practice of typing two spaces before a sentence terminator in order to create a larger break between sentences. Wigmore noted that this practice became common among writers in nineteenth-century America when word processors did not exist yet. He also said that using two spaces after a period makes sentences easier to read.
However, the two-space rule does not make sense for programs because you should not have any space at all between words in computer code. So, putting two spaces after a period serves no purpose other than to make reading printed material easier for human beings.
Because the typewritten monospace typeface has so much more spacing, writers who used typewriters needed the extra space after punctuation to denote a complete stop, such as a period, question mark, or exclamation point. Using two spaces will cause your typeset to be distorted.
As a result, the two-space rule was adopted—on a typewriter, an extra space after a sentence makes writing simpler to read. Typographers believe that because we've all shifted to current fonts, adding two spaces after a period no longer improves reading. It weakens it. So they recommend only one space after a sentence terminator.
Single sentence spacing was defined as a typographic norm in 1989 by Desktop Publishing by Design, which noted that "typesetting requires just one space following periods, question marks, exclamation points, and colons." Prior to this, multiple spaces were used to create more emphasis.
This changed in 2007 when the Chicago Manual of Style removed the requirement for single sentence spacing. Instead, it states that "periods should be spaced only if necessary to avoid disrupting the flow of the text" and gives as an example that "a single sentence can use no more than one space after the end of a paragraph."
For existing publications that you do not want to change, there are several workarounds. You can include three dots at the end of a sentence (...), or you can use ellipses ("..."). Both methods give the same result: one space after the period.
If you're creating new material for publication, then one space after the period is required.
(This is referred to as a mono-spaced typeface.) When using a typewriter, it makes sense to add an extra space to indicate that the sentence has finished. However, today's word-processing software makes typefaces proportional, so we only need one space. That's why it's important to set your computer's default font to contain only one space after a period.
When writing in a monospaced typeface, these two spaces are known as the period space and the paragraph space. They are used to separate complete sentences from each other, and also to indent new lines of text within a document.
Monospace fonts are useful when you want to align characters along the left or right margins (or both). Because there is no variation in line length, using a monospace font ensures that all words in a sentence are aligned with each other.
These days, most computers use Bitstream Vera Sans Mono as their default monospace font. It is a modern, compact font that looks great printed out on paper, and it's available for free online.
The term "monospace" comes from the fact that these fonts vary in size only at the end of lines. All other letters in a given word take up the same amount of space, which means that they are aligned with other words in a sentence.
However, word processing software may readily solve this problem by maintaining the space between characters to a minimum. And it's not only an editorial preference (though it is entirely an editorial preference), but the main style guides all agree that one space, not two, follows the period.