Natural gut strings, made from cow intestine (yes, really). The original tennis string and still considered the best for feel, power, and arm comfort. They hold tension better than synthetic strings. The downside? They're expensive (around $40-50 for a set) and less durable, especially in wet conditions. Federer famously used a natural gut and polyester hybrid setup throughout his career. Many pros still use gut in some combination.
"Full bed of natural gut at 55 pounds. Beautiful feel, beautiful sound, and it'll probably break in two matches."