What is the scientific name of writing couplets?

Zheng Dan paper dyed with red lead is not easy to fade. The red lead used is polished, colored with glue, dyed by two people, and baked with charcoal fire. When baking, the fire surface is covered with grass ash to prevent the paper from being damaged by flame. This paper is better with jade buckle paper. Fine workmanship, strict requirements and low output. In the past, only 400~500 sheets were dyed per person per day. This kind of paper was generally used for more delicate articles, such as book covers or titles of books and periodicals. China ancient book thread-bound books all use this paper as the title page. In addition to decoration, it also has the function of moth prevention and has special effects on protecting books. Bright red does not fade easily. Sprinkle silver foil or copper-tin foil on Zheng Dan paper, which is called "silver doping", which is more distinctive. "Mixed silver" positive Dan paper is often used as cover or couplet. Because it is not easy to fade for a long time, it is called "Wan Nianhong" or "Never Fade".