Linen thread is considered the strongest and highest quality sewing thread used for bookbinding. This thread comes on a large 100 gram spool (~250 yards). When using 10/3 thread, we recommend using the #1 needle. The 10/3 thread is approximately 0.75 mm in thickness. It used where an exceptionally strong thread is desired, or for exposed sewing structures.
With threads, the first number is the gauge (i.e. thickness) the second number is the number of strands or ply. Like electrical wire, the larger the gauge the thinner the thread. A 25/3 thread and an 18/3 thread are both 3 ply, but the 25/3 is overall thinner. 25/3 is a standard in-between thickness and the most popular for hand bookbinding.