In an ideal assembly design, every component of a product should be part of a frontline process: the bill of materials (BOM) should be a single-level list of items (‹at), and any subassemblies, if necessary, should be built in-line as needed (vertically integrated). Unfortunately, it is not always practical or feasible to put everything in one line or cell. Subassembly feeder lines are sometimes needed to supply parts to a ­nal assembly line.