It is important to give due consideration to the measurement xx/0 still in use today for most of the fly tying threads, measurements which in the past created confusion regarding the different threads available on the market and that, overall today, is subject to misleading communication, especially for the end user.

The xx/0 system is absolutely not recognised as a legal measurement to indicate the thickness of threads, it is solely an arbitrary measurement given by the tying thread manufacturers. The xx/0 measurements is derived from an old system of measuring silk threads which combines a series of 00’s to indicate the fineness of the thread, the more 00’s assigned, the finer is the thread. This is where this trend originated, for example a 3/0 indicates a thick thread, an 8/0 a finer thread and a 12/0 a finer thread.

A more accurate and legally recognised system for the thread measurement is the Denier, which indicates the weight in grams for every 9000 meters of thread. The Denier together with Decitex is also used universally by the synthetic thread manufactures in the textile industry and is always indicated in all the documents which accompany the thread during its process, in some cases also shown in the final product (for example in women’s tights). Practically a thread marked denier 70 is finer than one marked Denier 100 but in fact it is exactly half when compared to one of Denier 140.

There are obviously other measuring systems universally recognised in the textile industry like Decitex (already mentioned), the Tex, the Metric Count and and the English Cotton Count, even though they are used in diverse categories and products they all refer to the thread value attributed to its weight and length or to the length versus its weight.

Many producers who supply thread for the fly tying market also give information regarding the Denier of their threads, either on the spool itself, via their website or in the brochures one can find in the shops, technical details which should be indicated as a must – not as an option.


Synthetic fibres, defined also as man-made fibres, which came in use in the 1930’s, have been produced from organic matter such as petroleum, thanks to a chemical reaction of polymerisations.
The two main synthetic fibres used for the thread production are polyester and polyamide, which have similar characteristics and the same breaking strength; there are only a few particular polymers, which have a higher breaking strength.
Between these two principal distinctions, one can affirm with certainty that threads with similar Denier have similar breaking strength.

One has to mention also the high-tech synthetic threads like Dyneema and Spectra (Gel spun polyethylene fiber) or Kevlar and Twaron (para-aramidic fibers), which all have a very high breaking strength, from four to five times superior to any other thread with similar Denier.
Being a highly technological material, the costs for this product is much higher compared to the normal polyester and polyamide.

Obviously there are also other synthetic fibres which have not been mentioned in this short report since they have values of breaking strength and usability that are not suitable for the fly tying or that have a limited usage for specific patterns.