Commodity curves (which plot time vs. percent complete, also called line-of-balance or velocity charts) do not reveal, however, what resources were needed to adhere to the planned installation schedule. Their usefulness is limited when managing unique materials because they do not help recognize matching problems. This is the case here as pipe spools really are not commodities. "They [spools] differ in specification requirements, physical configuration, and adjacent plant features. These important differences make both establishing performance standards and comparing results very difficult" (Tatum 1985).