The Brass Reducing Flare Nut is used to maximize strength and eliminate cracks Tubing should be double flared in extreme applications to minimize flare thinning on over-tightening. Flare nuts are also known as tube nuts or line nuts. They are available in a wide variety of sizes and materials to accommodate different types of tubing, fittings, and applications. Flare fittings are a form of compression fitting that is commonly used with metal tubing. Brass Reducing Flare Nut is very effective.