The Period, Full Stop or Point
The period (known as a full stop in British English) is probably the simplest of the punctuation marks to use. You use it like a knife to cut the sentences to the required length. Generally, you can break up the sentences using the full stop at the end of a logical and complete thought that looks and sounds right to you.
Mark the end of a sentence which is not a question or an exclamation
Rome is the capital of Italy.
I was born in Australia and now live in Indonesia.
The Dalai Lama is the spiritual leader of the Tibetan people.
Indicate an abbreviation
Many abbreviations require a period. Dr, Mr, Mrs, and Ms do not take a period in British English, nor do most abbreviations taken from the first capital letters such as MA, Phd, or CIA. In American English, some of these do require periods or both usages are correct (with and without periods). If you require 100% accuracy in your punctuation, refer to a detailed style guide for the abbreviation usage rules in the variety of English you are using.
I will arrive between 6 a.m. and 7 a.m.
We are coming on Fri., Jan. 4.
Often you will see a sentence concluding with three dots. This indicates that only part of the sentence or text has been quoted or that it is being left up to the reader to complete the thought.
The Lord’s Prayer begins, “Our Father which art in Heaven…”
He is always late, but you know how I feel about that…
Period after a single word
Sometimes a single word can form the sentence. In this case you place a fullstop after the word as you would in any other sentence. This is often the case when the subject is understood as in a greeting or a command.
Periods in numbers
Numbers use periods in English to separate the whole number from the decimal. A period used in a number is also called a “decimal point” and it is read “point” unless it refers to money.
$10.43 = ten dollars and 43 cents
14.17 = fourteen point one seven