Because of its strength, linen thread is used to sew signatures in bookbinding. It is often lightly waxed with beeswax (S289).12/3, 18/3, 35/3.
The first number is the thickness of the thread. The higher the number, the thinner the thread. That is 12/3 is thicker than 18/3. The second number is the number of strands that are twisted together to make the thread. 12/3 has three strands. (Flax and linen are two words for the same thing = fabric or thread made from the flax plant.)