When do we need a period to indicate an abbreviation and when don't we?<br/><br/><i>Mr (mister)</i><br/><br/><i>Dr (doctor)</i><br/><br/><i>St (saint)</i><br/><br/><i>intr (intransitive)</i><br/><br/><i>etc</i><br/><br/>Thanks a load.