Blueprint drawings (as used in the building industry) usually indicate how a structure, item, or system is to be built, implemented, updated, or repaired. One of the primary purposes of graphic symbols on construction drawings is to relate to other drawings in the set. For example, arrows are used to show direction and connection between drawings; scales are used to establish size relationships between objects shown on different drawings; and symbols for special conditions can alert others to make necessary adjustments to other drawings. The first drawing in a set of related drawings should include all relevant information needed by those constructing the finished product. This first drawing is often called a "master" or "parent" drawing. It may include the overall size of the project, the materials required, the methods used to construct items such as windows and doors, the types of connections between elements, and any other information needed to complete the job. The second drawing in the set is typically a "detail" drawing showing one section of the main structure or element being constructed. Detailed drawings should include information about specific materials used in the construction process, names of contractors who will be working on the project, locations of materials and equipment within the building site, and any other details that will help in completing the job. A third drawing may be required if there is further clarification needed from the master or parent drawing.