Final answer:
The strength of threads used for sewing is determined by the thread's thickness or weight, the type of fiber used, and the construction of the thread.
Step-by-step explanation:
When considering the strength of threads used for sewing, it is determined by the thread's thickness or weight. Thread strength is typically measured in terms of thread denier, which refers to the weight in grams of a 9,000-meter length of thread. The higher the denier, the stronger the thread. For example, a thread with a denier of 100 is stronger than a thread with a denier of 50.
Another factor that can determine thread strength is the type of fiber used. Different fibers have varying tensile strengths, which impacts the overall strength of the thread. For instance, polyester and nylon threads are generally stronger than cotton threads.
Additionally, the construction of the thread can affect its strength. A thread with multiple plies or strands twisted together will be stronger than a single-ply thread. The number of twists per inch (TPI) also affects strength, with a higher TPI generally indicating greater strength.