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.
In printed material, where everything is set in type, only put a space after the period if it makes a difference to the meaning of the sentence. That's usually only necessary with abbreviations (typesetters don't like themany periods), contractions (don't use one space after a contraction) and quotations (one space indents the text).
Between paragraphs, leave a gap instead. The idea is that each paragraph is its own unit, so there should be no link between them other than what they share as common boundaries. Paragraphs are usually identified by indenting the first line or by using punctuation such as quotes or headers. These identify the start of a new thought or section of the document and should not be connected to others.
When writing for an audience that will read this out loud, it's important to give enough time between sentences for the reader to pause for breath. This allows them to connect the ideas in your sentence properly and gives the impression that you're not talking at them but rather about them. So, two spaces is appropriate when reading out loud.
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, one space after a period is acceptable but two spaces are still commonly used in formal writing to indicate that this is a new paragraph.
The reason for this is that computers based text editors rely on punctuation marks to tell them where one sentence ends and the next begins. If there's no punctuation mark between sentences, the editor has no way of knowing when one sentence ends and the next one starts. This can cause problems when editing your work later. For example, if you delete the last sentence in a paragraph and don't add any punctuation, then the editor has no way of knowing where the end of the paragraph is so it will keep the whole thing.
By using two spaces after a period, you're telling the editor that you want this sentence to be part of the same paragraph as the previous one.
This isn't necessary in informal writing but shows respect to your readers who may not understand why punctuation is important in a computer-based medium.
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. This is why writers today should not use a double space after a period.
He argues that the practice of two spaces between sentences dates back to the days when everyone typed on manual typewriters. Because of how typewriters handled proportional spacing, typewritten manuscripts with two periods between sentences were simpler to read. This idea is supported by the fact that many early typewriter keyboards lacked an option for single punctuation marks.
Today, most lawyers use computer-generated documents. One space represents one pause in speech, so two spaces between sentences looks odd to readers used to seeing only one space. The problem is avoided by some writers, editors, and printers by inserting a third space after the second period. This makes it possible to have two-space paragraphs without looking like you're trying too hard or like you've got something to hide.
The two-space rule can be found in almost all types of writing, but it's particularly common in legal documents. Some examples of other fields where you'll often find two-space paragraphs include science journals, academic papers, magazine articles, brochures, flyers, newsletters, and even advertising materials sometimes require two spaces instead of one.
There are several reasons why lawyers might use two spaces instead of one. First, it allows them to spread out their thoughts within a sentence without appearing overbearing or lacking in confidence. Second, it helps readers understand that there is more than one idea within the sentence.
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. Examples include periods, commas, and semicolons.
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 writing easier to read.
In print, the two-space rule still applies but editors often use only one space instead. The rationale for this change over time has been that using no space at all makes the text look too tight and uncomfortable to read. However, starting with Microsoft Word, it is possible to have single spacing as well.
The two-space rule remains in effect on typewriters because without this rule, typing a sentence would be impossible: You would need a break of some kind after every letter you typed. This isn't practical for writing that gets printed out.
In electronic files, the two-space rule doesn't make much sense anymore since there can be no gap between words without breaking the text into separate sentences. But if you edit these files manually (using a word processor or not), editors usually leave a space after each sentence regardless of whether it fits on a page by itself.
For web pages, using two spaces after each sentence makes sense and gives the reader a chance to absorb the information within the sentence before moving on to the next one.