fact for those who have experience with them, plain gut strings are quite stable from a tuning standpoint. Wound gut has more instability of tuning due to the different response to moisture and heat between the winding and the core and from string to string. Some players use olive oil on gut strings to extend their playing life and improve tuning stability by reducing their sensitivity to humidity. Gut strings tend to hold their sound quality nicely right up until they fail or become excessively worn. Modern strings are most commonly either a stranded (arranged in single thin lengths twisted together) synthetic core wound with various metals, or a steel core, which may be solid or stranded, often wound with various other metals. With low-density cores such as gut or synthetic fiber, the winding allows a string to be thin enough to play, while sounding the desired pitch at an appropriate tension. The winding of steel strings affects their flexibility and surface properties, as well as mass. Strings may be wound with several layers, in part to control the damping of vibrations, and influence the "warmth" or "brightness" of the string by manipulating the strength of its overtones. The core may be synthetic filaments, solid metal, or braided or twisted steel filaments. The uppermost E string is usually solid steel, either plain or wound with aluminium in an effort to prevent "whistling." Gold plating delays corrosion of the steel and may also reduce whistling. Stainless steel gives a slightly different tone. Synthetic-core strings, the most popular of which is Perlon (a trade name for stranded nylon) combine some of the tonal qualities of gut strings with greater longevity and tuning stability. They are also much less sensitive to changes in humidity than gut strings, and less sensitive to changes in temperature than all-metal strings. Solid-core metal strings are stiff when newly replaced and tend to go out of tune quickly. While some gut strings still use a knot to secure the tail end in the slot of the tailpiece, most modern strings use a "ball", a small bead often made of bronze. A frequent exception is the E string, which may be had with either a ball or loop end, since the smallest E-string fine tuners hold the tail of the string on a single small hook. The price of different string types varies dramatically; gut and gut-core strings are