I am often asked about the how threads are measured and sized. Old school is based on an "aught" system. From largest to smallest you most common sizes are 3/0, 6/0, 8/0, 12/0, and 17/0. This also correlates with strength. Generally the larger the diameter of thread the stronger the thread.
A new method of sizing and labeling threads has come about in the past ten years and it is called Denier. Common denier sizes (from largest to smallest) are 280, 210, 140, 90, 70, and 40. The same holds true that a larger thread is generally, but not always, stronger. The nice thing about the denier system is that it makes more sense to most people. It is easy to remember larger numbers represent larger/stronger thread and vise versa. But what the heck is a denier.
According to Wikipedia, 'Denier or den is a unit of measure for the linear mass density of fibers. It is defined as the mass in grams per 9,000 meters. In the International System of Units the tex is used instead. The denier is based on a natural standard: a single strand of silk is approximately one denier. A 9,000-meter strand of silk weighs about one gram. The term denier is from the French denier, a coin of small value (worth 1/12 of a sou). Applied to yarn, a denier was held to be equal in weight to 1/24 of an ounce". So, it is basically based on the weight of the thread.
When I attempt to answer the above question it is usually followed up with, "what aught size equals what Denier"? Fortunately, the Wapsi company has already done the dirty work in developing a table that shows some of the major thread brands their sizes and the Denier equivalent.
Keep in mind that more wraps of a smaller diameter thread are stronger then a few wraps of a large thread. All threads have their application but you do not need to have all sizes on hand to tie most flies. Decide on what manufacturer you like and what size you are most comfortable with and enjoy fly tying.
