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Elements of a flow or circuit diagram, grouped because each one is a picture of a mechanism rather than a label for it. The check valve draws the ball against its seat so the one-way direction is visible from the geometry; the heat exchanger draws a long path in a small space, which is the operating principle; the relay draws its coil and contacts apart because they genuinely are. Reading these is reading the drawing rather than remembering a legend.
Two triangles meeting at their points, the generic valve on a flow diagram.
A ball or flap against a seat, permitting flow one way only.
A circle with a triangular wedge, marking a pump and its flow direction.
A circle on a stem with a pointer or letters, marking a pressure indicator.
A circle or rectangle crossed by a zigzag, marking heat transfer between streams.
A flat-backed D shape, outputting true only when every input is true.
A curved-back shield shape, outputting true when any input is true.
A coil beside a switch, showing a circuit that a second circuit operates.
A resistor with an arrow touching it, marking a variable division of voltage.
A small rectangle between two plates, marking a quartz frequency reference.