Most flute keys have needle springs, made of phosphor bronze, stainless steel, beryllium copper, or a gold alloy. The B thumb keys typically have flat springs. Phosphor bronze is by far the most common material for needle springs because it is relatively inexpensive, makes a good spring, and is resistant to corrosion. Unfortunately, it is prone to metal fatigue. Stainless steel also makes a good spring and is resistant to corrosion. Gold springs are found mostly in high-end flutes because of its cost.[citation needed]