Linen thread is considered the strongest and highest quality sewing thread used for bookbinding. It comes on a 50 gram spool (~225 yards). When using 25/3 thread, we recommend using the #3 needle. A 25/3 thread is approximately 0.50 mm in thickness.
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.