Understanding Thread Measurement in Fly Tying: Aught vs Denier

Fly tying is an art that requires precision and knowledge of materials, and one of the crucial aspects of this craft is selecting the right thread. Two common systems used to measure thread thickness are the Aught and Denier systems. Understanding these systems is key to choosing the right thread for your fly tying projects.
The Aught System
The Aught system, denoted as "/0" (e.g., 6/0, 8/0), is a traditional method used primarily in fly tying. In this system, the higher the number before "/0", the thinner the thread. For example, 6/0 is thicker than 8/0. However, it's important to note that the Aught system is not standardized. The thickness for a given aught size can vary between manufacturers. This system is more about relative thickness and is deeply rooted in the tradition of fly tying.
The Denier System
On the other hand, the Denier system offers a more scientific approach. Denier measures the linear mass density of fibers and is defined as the mass in grams of 9,000 meters of thread. A higher denier means a thicker thread. This system is widely used in the textile industry and provides a consistent and quantitative measure of thread thickness. For instance, a 70-denier thread is thinner than a 210-denier thread.
Comparing Aught and Denier
To better understand how these systems correlate, let’s look at a chart comparing Aught sizes with Denier values:

Aught vs Denier

As illustrated, the relationship between Aught size and Denier is inverse; as the Aught size decreases (toward 1/0), the Denier value increases, indicating a thicker thread.
Why This Matters
For fly tiers, selecting the correct thread size is crucial for the type of fly being tied. Smaller flies require finer threads (higher aught number or lower denier), while larger flies need thicker threads (lower aught number or higher denier). The choice of thread affects not just the appearance but also the functionality of the fly.
Conclusion
Understanding the Aught and Denier systems is essential for fly tiers of all levels. While the Aught system remains popular due to its historical use in fly tying, the Denier system provides a more precise measurement. By familiarizing themselves with both systems, fly tiers