Linen thread is considered the strongest and highest quality sewing thread used for bookbinding. This thread comes on a a large 100 gram spool (~250 yards). When using 10/3 thread, we recommend using the #1 needle. The 10/3 thread is approximately 0.72 mm in thickness. This thread is often used in exposed sewing artists books and for other purposes where an exceptionally strong thread is desired.
With all our threads, the first number is the gauge (i.e. thickness) the second number is the number of strands or ply. Like with 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.