to show only one of them on the drawing, and to define the symmetrical counterpart simply by stating that it is "opposite". Common notations include "left-hand shown, right-hand opposite" or "-1, LH (shown); -2, RH (opposite)". Phantom parts The term phantom part is sometimes used to describe a series of parts that collectively make up an assembly or subassembly. This concept is helpful in the database management of engineering and production (such as in product data management applications) when it is useful to think of a certain combination of subparts as "one part" (and thus one database record) for ordering, production, or billing purposes. Synthetic parts It is common in the engineering of parts, subassemblies, and higher assemblies to treat the definition of a certain part as a very well-defined concept, with every last detail controlled by the engineering drawing or its accompanying technical product documentation (TPD). This is necessary because of the separation of concerns that often exists in production, in which the maker of each part (whether an in-house department or a vendor) does not have all the information needed to decide whether any particular small variation is acceptable or not (that is, "whether the part will still work" or "whether it will still fit into the assembly" interchangeably). The sizes of fillets and edge breaks are common examples of such details where production staff must say, "it may easily be trivial, but it could possibly matter, and we're not the ones who can tell which is true in this case". However, a challenge to this paradigm (of perfectly frozen part definition) is that sometimes it is necessary to obtain a part that is "mostly like" part A but that also incorporates some of the features of parts B and C. For example, a new variant of model of next-higher assembly may require this. Although this "blending" of part designs could happen very informally in a non-mass-production environment (such as an engineering lab, home business, or prototyping toolroom), it requires more forethought when the concerns are more thoroughly separated (such as when some production is outsourced to vendors). In the latter case, a new part definition, termed a synthetic part (because its definition synthesizes features from various other parts), is created. Ideally it is then formally defined with a new drawing; but often in the imperfect reality of the business world, to save time and expense, an improvised