A piping and instrumentation diagram (P&ID) is defined as follows:
# A diagram which shows the interconnection of process equipment and the instrumentation used to control the process. In the process industry, a standard set of symbols is used to prepare drawings of processes. The instrument symbols used in these drawings are generally based on International Society of Automation (ISA) Standard S5.1
# The primary schematic drawing used for laying out a process control installation.
They usually contain the following information:
* Mechanical equipment, including:
**Pressure vessels, columns, tanks, pumps, compressors, heat exchangers, furnaces, wellheads, fans, cooling towers, turbo-expanders, pig traps (see symbols below)
**Bursting discs, restriction orifices, strainers and filters, steam traps, moisture traps, sight-glasses, silencers, flares and vents, flame arrestors, vortex breakers, eductors
*Process piping, sizes and identification, including:
**Pipe classes and piping line numbers
**Flow directions
**Interconnections references
**Permanent start-up, flush and bypass lines
**Pipelines and flowlines
**Blinds and spectacle blinds
**Insulation and heat tracing
*Process control instrumentation and designation (names, numbers, unique tag identifiers), including:
**Valves and their types and identifications (e.g. isolation, shutoff, relief and safety valves, valve interlocks)
**Control inputs and outputs (sensors and final elements, interlocks)
**Miscellaneous - vents, drains, flanges, special fittings, sampling lines, reducers and swages
*Interfaces for class changes
*Computer control system
*Identification of components and subsystems delivered by others
P&IDs are originally drawn up at the design stage from a combination of process flow sheet data, the mechanical process equipment design, and the instrumentation engineering design.  During the design stage, the diagram also provides the basis for the development of system control schemes, allowing for further safety and operational investigations, such as a Hazard and operability study (HAZOP). To do this, it is critical to demonstrate the physical sequence of equipment and systems, as well as how these systems connect.
P&IDs also play a significant role in the maintenance and modification of the process after initial build. Modifications are red-penned onto the diagrams and are vital records of the current plant design.
They are also vital in enabling development of;
*Control and shutdown schemes
*Safety and regulatory requirements
*Start-up sequences
*Operational understanding.
P&IDs form the basis for the live mimic diagrams displayed on graphical user interfaces of large industrial control systems such as SCADA and distributed control systems.