A first consideration must be made regarding the xx/0 measurement still used today for many threads employed in fly tying, a measurement that has already created much confusion in the past regarding the differentiation of products on the market and which, especially today, is the subject of misleading commercial campaigns, particularly for the end consumer.

The xx/0 system is absolutely not recognized as a legal measurement for indicating thread size; it is therefore a measurement arbitrarily assigned by thread producers for fly tying. The xx/0 measurement comes from an old silk thread measurement system that assigned a series of zeros to indicate the fineness of the thread: the more zeros assigned, the finer the thread. Hence the habit of indicating, for example, a 3/0 as a thick thread, an 8/0 as a thinner thread, and a 12/0 as an even thinner thread.

A more accurate and legally recognized system for measuring threads is the Denier, which indicates the weight in grams of 9,000 meters of thread. The Denier, along with the Decitex, is also universally used by synthetic thread producers in the textile industry and is always indicated on all documents accompanying the thread throughout the textile supply chain, in some cases even on the finished product (for example, in women's tights). As a practical example, a thread marked Denier 70 is 30% thinner than one of Denier 100, while it is exactly half compared to one of Denier 140.

There are obviously other universally recognized measurement systems in the textile field such as the Decitex, already mentioned, the Tex, the Metric Count, and the English Cotton Count; although used in different areas and for different product categories, they all refer to the thread with a value obtained from the ratio between its weight and length or between its length and weight.

Many fly tying thread producers also always provide information regarding the Denier of their thread, either on the spool itself or on their website or catalog available in stores. This technical and commercial detail should be indicated as a rule, not as an option.

Synthetic fibers, also called Technofibers, were discovered and developed around the 1930s. They are produced by spinning polymers obtained from compounds of organic origin, such as petroleum, through chemical polymerization reactions.

The two main synthetic fibers used for thread production are polyester and polyamide, which have similar characteristics in terms of breaking strength; only certain high-tenacity polymers have superior resistance. Within these two main categories, it can be stated with certainty that threads with similar Denier values will have similar breaking strengths.

A special mention must be made for high-technology synthetic threads such as Dyneema and Spectra (Gel spun polyethylene fibers) or Kevlar and Twaron (para-aramid fibers), which have extremely high breaking strength values, four to five times greater than any other synthetic thread of the same Denier. Being very technical materials, their cost is also infinitely higher compared to regular polyester or polyamide.

There are obviously other synthetic fibers not mentioned in this brief overview, as they have intrinsic resistance and usability characteristics not suitable for use in fly construction, or that find use but are particularly limited to specific applications.