dependends on the masonary being used of corse and of the type of dimension wanted- co-ordinating dim. or brick/block/unit size? and then if it is co'ordinating dim. the joint width has to be considered. Birkhausers Masonary Construction Manual is a very good book like al their construction manual series.

http://www.springeronline.com/sgw/cda/frontpage/0,11855,5-40463-72-34416306-0,00.html

basically it means that your dims are based off 4" or 8" so you don't have to cut units. Example, wall is 64'-4" long and 12'-8" high and your door rough openings are 3'-4" by 7'-4" - this is why you usually see 4" heads on doors in masonry, they course out.

also to note there are nominal widths and actual widths. use nominal - 8", 4", 16", 12" - when drawing plans. when drawing sections, plan details - use actual - 7 5/8", 3 5/8", 15 5/8", 11 5/8".......make sure your masonry openings are to nominal dimensions.......