In pneumatic diagrams, what typically represents control devices?

In pneumatic diagrams, control devices are typically represented by symbols. These symbols provide a standardized way to depict various pneumatic components, such as valves, actuators, and sensors, making it easier for individuals to interpret the functions and relationships between different elements in the system.

Symbols allow for quick identification and understanding of how specific control devices interact within the pneumatic circuit. Each symbol conveys specific information about the device's operation—whether it is normally open or closed, its function, or additional characteristics. This visual representation streamlines communication among engineers, technicians, and operators who work with pneumatic systems.

The other options, while part of the diagram, do not serve this specific purpose. Circuits refer to the complete pathways through which air flows, blocks imply segments of the diagram without detail on control functions, and lines generally indicate connections or fluid flow but do not provide information about the nature of control devices. Thus, symbols stand out as the most appropriate representation of control devices in pneumatic diagrams.