Abbreviation
In British English it is not necessary to indicate an abbreviation with a full stop (period) after the abbreviation, when the last letter of the abbreviation is the same as the unabbreviated word, while the opposite holds true in North American English. This means that while the abbreviation of Doctor is usually written as "Dr" in most of the Commonwealth, it is usually written as "Dr." in North America.
Similarly, conventions regarding the punctuation of degree abbreviations vary. In the United Kingdom, it is increasingly common to omit punctuations from abbreviations that are not truncations: while the usual abbreviation of "Esquire" is "Esq.", the usual abbreviation for "Doctor of Philosophy" is "PhD". It is not incorrect to use the fully punctuated "Ph.D.", though if this pattern is used, it should be used consistently; practice in particular situations may vary, and it is always more elegant to be consistent with local patterns of usage than to deviate from them.
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