A bicycle rim is connected to the hub by several spokes under tension. Modern bicycle wheels almost exclusively use spokes than can only be loaded in tension.
At the end of each spoke is a specialized nut, called a nipple, which is used to adjust the tension in the spoke. The nipple is usually located at the rim end of the spoke. The use of aluminium nipples at the rim reduces the moment of inertia, but they are less durable than brass. A third alternative is titanium nipples, which are extremely strong, but substantially lighter than brass. A nipple at the rim of a wheel usually protrudes from the rim towards the center of the wheel, but in racing wheels may be internal to the rim, offering a slight aerodynamic advantage.