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There is an old typographical convention, which obliges to put two spaces after the period in the sentence. The fact is that printing with one space was too continuous and the double space between sentences divided the text and made it more readable.
Nowadays, one space between sentences has become the norm, both for texts in electronic form and for printed copies. The format associated with the past era of thundering typewriters has now been replaced by using one space after each sentence.
The History of the Double Space
Putting two spaces each time after the period has been studied in typewriting lessons in the USA since the beginning of the last century, and this tradition is still alive. It is the habit of Americans who are now at least 45 years old. They remember the days when they were learning to type texts using an IBM Selectric typewriter. According to the rules that they assimilated, the double space should also be used after the interrogative and exclamation marks, the colon and the semicolon. The main purpose of using two spaces after a period was to increase the space between sentences. The font of that time was quite disproportionate, and the entire written text looked like a single whole. That formatting was unreadable.
Modern Typing Rules
When computers text editors appeared, huge opportunities for using proportional fonts opened up, in which, for example, the “thin” letter I does not take as much space as the “thick” W one, and the letters firmly hold each other within a single word. It is argued that when typing in a proportional font, the borders not only between words but also between sentences are more noticeable, which means that a double space is not needed; moreover, it is considered an error.
However, there is a recent study, which suggests that using two spaces after a period can improve the perception of the text and simplify the reading process. The effect of spaces is not significant. However, not all scientists trust this research and do not believe in its authenticity. That is why the use of two spaces at the end of a sentence is considered obsolete today. Although this question remains controversial, progress is still evolving, and people will forget about the double space soon. The texts with such formatting will be found only in museums.