In computer-aided plotting hardware, pen plotters have historically been implemented in three mechanical formats. Two are well known: flatbed and drum. What is the third common pen plotter mechanism used to advance media while the pen draws?

Introduction / Context:
Pen plotters are electromechanical devices that physically move a pen over media to draw vector graphics. Classic implementations are categorized by how the paper (or the pen carriage) is moved. Two widely cited types are flatbed plotters (paper is fixed on a flat surface; the pen moves in X and Y) and drum plotters (paper wraps around a rotating drum to provide one axis while the pen carriage provides the other). A third, also common in roll-fed designs, uses pinch rollers (often with grit wheels) to advance media precisely under the pen.

Given Data / Assumptions:

• We are specifically discussing pen plotter technologies, where a physical pen draws.
• Flatbed and drum are already listed as two of the three categories.
• We must identify the additional mechanical media-handling approach used historically in pen plotters.

Concept / Approach:
The third mechanism is the pinch roller or grit-wheel system. In these plotters, pairs of rollers press the paper against coated drive wheels. As the wheels rotate, the friction and the grit texture transport the paper forward or backward with fine control, while a pen carriage moves laterally. This allowed long, roll-fed plotting with accurate step movement without a full drum assembly.

Step-by-Step Solution:

Verification / Alternative check:
Product literature and historical overviews of plotters distinguish flatbed, drum, and pinch-roller/grit-wheel categories for pen devices. Electrostatic and thermal are different printing technologies, not pen plotters.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing printing technologies with pen plotter mechanisms; overlooking that pinch-roller systems are still pen-based but roll-fed rather than drum-mounted.

Final Answer:
Pinch roller (grit-wheel) mechanism.