Because the typewritten monospace typeface has so much more spacing, authors 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.
Monospaced type produces text that seems "loose" and uneven; there is a lot of white space between letters and phrases, making it more difficult to notice the gaps between sentences instantly. As a result, the two-space rule was adopted on a typewriter; an extra space following a sentence makes the text easier to read. This is known as "the pause rule."
In modern times, monospace fonts are used in programs that require precise formatting, such as print documents. The pauses between words in monospace text make it easy to spot errors when proofreading draft documents before final production.
The two-space rule applies to all languages that use the Latin alphabet, including English. Other cultures may use different amounts of space between words: 1/4 of a space in German, 1/8 in French, or no space at all in Arabic.
When typing English words without spaces (for example, for email messages), it is common practice to leave a space after each word to indicate the end of a thought. Some readers find this style of writing hard to follow because they have to wait until the last word is typed before starting to read. Others appreciate not having to stop every time someone stops thinking about what they're typing.
As a result, the two-space rule was adopted—on a typewriter, an extra space following a sentence makes the writing easier to read.
In print, where every bit of real estate is important, only one space after a sentence terminator is allowed. However, in online text, where you can make use of all that open space, two spaces after a sentence terminator is preferred.
The single-space version of your manuscript will be treated as though it were submitted with two spaces after each sentence terminator. The double-space version of your manuscript will be returned with no changes made other than replacing the two-space rule with the single-space rule.
The reason for this difference is simple: Readability. With two spaces after each sentence, the reader can follow the line of thought within the text more easily because they have less distance to travel between sentences. This is especially important when reading long documents or articles on multiple topics. By contrast, using one space after each sentence makes for more readable text if you want to include several sentences from different parts of the document in one argument.
Two spaces after a sentence terminator is common in formal writing, such as academic papers and legal briefs. One space is used in most other contexts.
Typographers believe that because we've all moved to current fonts, adding two spaces after a period no longer improves reading. It weakens it. Therefore, one space is used instead.
Unless you're using a real typewriter, there's no need to place two spaces following a period. Alternatively, a question mark. Alternatively, an exclamation point. This rule applies to all types of end punctuation.
The standard typewritten page has 90 characters in it, so if you are typing at a reasonable speed of 100 words per hour, you can type a full page every three hours. That's not very long! A standard computer keyboard has 64 keys, which means that you can type either 2 pages or 32 pages depending on how fast you work.
A word processor will usually give you the option of typing single or double spaces after periods and other punctuation. If you choose single spaces, only single spaces will be used when you save your file. If you choose double spaces, both single and double spaces will be used. Use one or the other but not both.
Some people say you should never use single spaces because it looks like you didn't finish your thoughts out loud, but most writers don't worry about appearances. They just want to get their ideas down as quickly as possible without thinking too hard about perfect spelling and grammar. If you are one of these people, then using single spaces is fine, but for the majority of us who care about our writing, we should use double spaces.