Half round is used to create flush mount columns.  All components are matched by series, depending on the size of the column itself. 

Series 100:   columns are 1" wide and are to be used where the fill width is 1-7/8" or wider.

Series 150:   columns are 1-1/2" wide and are to be used where the fill width is 2-1/2" or wider.

Series 200:   columns are 2" wide and are to be used where the fill width is 3" or wider.

Series 250:   columns are 2-1/2" wide and are to be used where the fill width is 3" or wider.