Here is the plain English of #header .callout: Select all elements with the class name callout that are decendents of the element with an ID of header. Here is the plain English of #header.callout: Select the element which has an ID of header and also a class name of callout. Maybe this graphic will make that more clear:
Wilmington, Ohio. Wilmington is a city in and the county seat of Clinton County, Ohio, United States. The population was 12,520 at the 2010 census.
Sponsored Topics. Wilmington is a town in Middlesex County, Massachusetts, United States. The population was 22,325 at the 2010 census. Wilmington was first settled in 1665 and was officially incorporated in 1730, from parts of Woburn, Reading and Billerica. Minutemen from Wilmington responded to the alarm on April 19, 1775 and fought at Merriam's Corner in Concord.