Linen thread is considered the strongest and highest quality sewing thread. The unwaxed heavyweight (18/3) linen threads are imported from Northern Ireland. They come on a 50 gram spool (~175 yards). 18/3 thread uses a #1 needle (larger eye and needle diameter) or #3 needle (smaller eye and needle diameter). Note: It is a little more difficult to thread the needle through the eye of a #3 needle, but the smaller diameter makes a smaller hole when sewing.
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 good in-between thickness and the most popular for hand bookbinding.