When a sentence is complete, use a period to mark its completion. A simple rule for period usage, but is it that simple?
Periods (or ‘full stops’) mark the termination of a sentence. For example,
Ashoka was a great emperor.
Simple enough. Note that the period is placed immediately after the last letter of the last word. The next sentence begins after one space. Looks obvious when I say it, but the rule is often ignored.
The article How To Combine Two Or More Sentences discusses what makes a complete sentence, and how we have to use conjunctions and punctuations to combine them.
First, let us differentiate between abbreviations and contractions. You must use a period for abbreviations,but not for contractions. Words are abbreviated by omitting their end parts, whereas contracted words retain the starting and ending letters of the word.
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Doctor has been contracted (its first and last letters appear) to Dr and does not require a period, whereas Professor has been abbreviated to Prof. and has been used with a period.
Again, what I am suggesting is British English. American English follows different rules. So, the Americans would use a period after every abbreviation; typically they write Mr., Mrs., Messrs., Dr., Sen., St., etc. Notice that Miss is not an abbreviation, so we don’t put a period after it. Ms. is not an abbreviation either, but a period is still used after it to keep, it consistent with Mr. and Mrs.
The use of periods in abbreviations is largely a matter of convention and, over time, the convention has increasingly moved to less use of periods in abbreviations.
The Use of the Period in Acronyms
Acronyms are abbreviations made up of the first letter from a series of words. We pronounce acronyms as words, not as series of letters, and they usually do not require periods: NATO, LASER, SCUBA, RADAR,UNICEF. Note that many style books are now recommending SMALL CAPS for all appearances of acronyms so that they blend smoothly with the rest of the text. Abbreviations are spelled out as letters and may or may not use periods and may be a question of convenion/style.
She doodles, runs her fingers through the hair constantly, etc., while talking to me, and it distracts me like crazy.
It distracts me like crazy when while talking to me she doodles, runs her fingers through the hair constantly, etc.
The teacher told the students to write, “A friend in need is a friend indeed.”
But if it is a partial quotation, then the period is not inserted within quotes:
When my friend came to my aid at the crucial time, I thanked this “friend in need”.
When the punctuation is not semantically associated with the quoted text, there is no good reason for it to be placed inside. In the following sentence, the question mark had to be placed outside, obviating the need for a period.
Was it not my teacher who used to say, “A friend in need is a friend indeed”?