Periods are the marks that appear at the end of a complete sentence. They are considered to be full stops, rather than pauses.
Rules that apply to the use of periods:
Software programs that use proportional spacing automatically add enough space after a period. Therefore, it is only necessary to hit the space bar once after typing a period. Some users are in the habit of hitting the space bar twice. This habit is left over from using a typewriter. Typewriters used manual spacing, meaning the space allotted for each letter was the same size. Many who learned to type on a typewriter were taught to hit the space bar twice after the period to create the correct amount of space. Doing this when using software programs such as Microsoft Word creates large blank spaces. Too many of the large blank spaces can cause what are called "rivers" in the body of the text. As the name suggests, the spaces cause the text to appear as if it there are clear paths winding through the text. From an aesthetic as well as space perspective, it is now correct to type only one space after a period at the end of a sentence.
Janie Sullivan, MBA, MAEd, has been teaching adult learners for over 20 years. She has taught online over 15 years, specializing in writing, communications, and small business applications. Janie directs the Center for Writing Excellence where she offers writing, editing, and formatting services for writers. She has been published in several newspapers and magazines as well as multiple online sites. She teaches communication, business strategy, leadership, and management courses. Janie has published a book "Develop and Deliver an Online Class." This is the third book she has written about writing and teaching online. She also has published a novel and an anthology of short stories.