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Parallel rule in plastic with aluminum arms lying on a cutting mat. Parallel rulers are a drafting instrument used by navigators to draw parallel lines on charts. The tool consists of two straight edges joined by two arms which allow them to move closer or further away while always remaining parallel to each other.
A pencil-beam radar A moving or sweeping pencil-beam radar. In optics, a pencil or pencil of rays, also known as a pencil beam or narrow beam, is a geometric construct (pencil of half-lines) used to describe a beam or portion of a beam of electromagnetic radiation or charged particles, typically in the form of a cone or cylinder.