Good morning!  Supplemental to your great DD post for anyone interested further in charts, legends, UOM, etc.  This how NOAA looks at it.  Enjoy! http://www.nauticalcharts.noaa.gov/mcd/learnn...dings.html

If this isn't enough for the curious, there is this source with additional links/sources. http://en.wikipedia.org/wiki/Defense_Mapping_..._.28DMA.29