Inking pen or liner or ruling pen

An inking pen, also known as a liner or ruling pen, is a precision drawing instrument used in engineering drawings.

According to the source material, its key characteristics and uses include:

• Primary Function: It is specifically designed to ink straight lines that have been previously drawn with other instruments.
• Prohibited Uses: It must never be used for freehand lines or lettering.
• Availability: It is a standard component found in an engineering drawing instrument box.
• Compass Attachment: It is often provided as a pen point attachment that can be inserted into the knee joint of a large compass to draw inked circles or arcs.

It is used to ink the straight lines drawn with the instruments but never for free hand lines or lettering.