Because the spacing between words on a typewriter was irregular, the extra space was required to define the beginning of a new phrase. If you didn't learn to type on a typewriter, you're probably using two spaces after the period because you're emulating the writing of someone who did.
Before word processors, two spaces after a period were necessary and taught as right if you learnt to type on a typewriter. This is called "paragraph formatting" in modern terms.
In today's world of computer-generated documents, two spaces after a period are not only unnecessary but also incorrect. The use of a single space is now assumed unless otherwise indicated by using the Escape sequence within a text file or application software.
The single space after a period is used in alphabetized lists because it avoids placing periods at the end of words when they belong to another list item. For example, "a.m." and "p.m." are separate entries in an alphabetic list. However, if there were no space between the hours and the minutes, then they would be listed together as one entry: "8 a.m."
Multiple spaces are used in quoted material or when writing an abstract because each line should be treated as a separate paragraph. A single space follows questions marks because they indicate a break in thought. Two spaces are used before a capital letter because this is how you identify important words or phrases.
Double spaces are used in written language that was created before computers came into use.
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.
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 advantage won the periods the status of one space.
In today's digital age, two spaces after a period are almost obsolete. The use of automated text formatting tools such as Word's "Bullet List" can easily produce documents with multiple paragraphs between each item in the list. When this is done, it is customary for the reader to insert another period to separate each paragraph. Since typing two periods is an easy task, many people omit the second period, which can cause problems later if you try to merge these separate paragraphs into one section or document break.
The American Bar Association recommends removing one space after a period in formal writing. This reduces the risk of including extra material in a manuscript by accident. A court reporter may also prefer not to type a second period because it could be interpreted as making part of the sentence incomplete.
However, some people still prefer to include two spaces after a period. They say it looks more like speech and gives the text a more informal feel.
Unless you're using a real typewriter, there's no need to place two spaces following a period. Alternatively, a question mark. Or an exclamation mark. This rule applies to all types of end punctuation. A period is the most common way to indicate the end of a sentence, so it makes sense that it would be followed by another period.
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 after a sentence makes writing simpler to read.
In print, instead, only one space is used between sentences. So, while typing a document you need to remember to add a second space after each sentence. Computers can't tell the difference between a sentence break and a blank line so they will assume that you want both spaces if you don't mark the break.
The one-two space rule dates back at least as far as Thomas Edison's phonograph in 1877. He claimed that his invention was "the greatest convenience in life about which anything has been said or done". His statement probably prompted many writers to add a second space after short sentences for clarity.
Even though most computers use one space as the default setting, some manufacturers have chosen to offer two space options as well. These special keyboards are called "text editors" and they allow you to edit entire documents without printing them first.
Some word processors allow you to change the space after sentences from the default single space to a double space. This feature should not be used by non-technical people because they might think that they are writing in manuscript format and use up all their ink when typing documents.