Most of the threads we use are round and smooth, as a result of twisting multiple strands together. We often think of thread consisting of only two or three strands (2-ply or 3-ply) twisted together. In the case of spun threads such as cotton or spun poly, it may be that simple, but with multi-filament threads such as polyester, silk, or rayon, it gets a little more complicated. A #50/3-ply polyester thread such as So Fine! #50 has three strands twisted together, but each of those three strands or plies are made up of many micro-strands twisted together. Less expensive threads may have a low number of micro-strands. High quality threads have as many as 48 micro-strands in a single ply.
Smooth fibers such as silk, multi-filament polyester, and rayon have a higher sheen than non-smooth fibers (such as cotton and spun polyester). However, even within the smooth fiber category, there is a wide range of sheen or luster. High sheen is the result of each individual micro-strand of fiber reflecting light, and the shape of the individual fibers determine how light is reflected.
Round fibers - Regular multi-filament polyester threads such as So Fine! #50 and Bottom Line fit into this category. As light hits the fibers, it is reflected away from our eyes, which results in a medium sheen appearance.
Trilobal fibers - Magnifico, Rainbows, Nature Colors, Living Colors, and Art Studio Colors threads are made from trilobal polyester fibers. Silk is a natural trilobal fiber. Trilobal fibers have three distinct sides('tri' = three and 'lobal' = sides), so when light hits them, they reflect back the light in a direct path, giving a high-sheen appearance. Although the individual micro-fibers are trilobal in shape, when multiple strands are twisted together to make the final thread, the end result is a smooth, round, high-sheen thread. Until recently, trilobal polyester fibers were weaker than regular round polyester fibers, so we had to choose between strength and high sheen. We now have both. A new high strength, high sheen trilobal polyester (Magnifico) is now available. It consists of 192 micro-strands of high tenacity (strength) trilobal polyester twisted into a smooth, round #40 thread.
Irregular fibers - Most rayon threads are irregular. We don't use or recommend rayon threads because they are weak and usually do not hold up to common standards of colorfastness.