Page "Abbreviation" Paragraph 8

Over the years, however, the lack of convention in some style guides has made it difficult to determine which two-word abbreviations should be abbreviated with periods and which should not.

The U. S. media tend to use periods in two-word abbreviations like United States ( U. S .), but not personal computer ( PC ) or television ( TV ).

Many British publications have gradually done away with the use of periods in abbreviations.