There are typically two shapes of ductwork that will be installed on any particular job. One is the standard square duct that we all have seen. The other is rounded duct. Both have pros and cons, and both will be better applied to certain types of jobs. Lets discuss.
Square ductwork is for us, manufactured in house. It typically is installed in places where the ductwork will be visible like a garage. This ductwork has a better aesthetic value with sharp angles and lines. The duct work is manufactured in 2 pieces and joined together at the job. The insulation is typically applied to the inside of the box and fastened with glue and welded tabs. This is not to say that this ductwork will not be installed in a crawlspace or attic, because it can. Ultimately, it will come down to the contractor and the homeowner.
Round ductwork is typically manufactured elsewhere and purchased at supply houses. This metal duct is wrapped with insulation on the outside with a wrap style insulation. Because of the outside wrap that is applied, this type of ductwork is typically not as aesthetically pleasing. Often times this ductwork is installed in places less visible like crawlspaces and attics. This application is a little more efficient due to the nature in which air flows.
Both applications work very well. One type will provide good angles and lines and the other will provide a slightly less than appealing but efficient alternative that will likely be out of sight!