A spoke nipple holds a bicycle wheel's rim to each spoke.
The spoke nipple is a headed cylinder with threading on the inside through part of its length (there is an unthreaded lead-in nearest the "flats"). The nipples will fit in holes in the rim, so that the head of the nipple is on the outermost part of the rim, while the other end of the nipple points inwards towards the hub. The threading grasps the spoke, so turning it increases or decreases tension in the spoke and influences the position of the hub relative to the rim.
Since spoke threading and nipple threading must match, boxes of spokes usually include a like number of nipples. Weight savings or the need for great strength due to building a wheel with few spokes might call for obtaining different nipples than those shipped with the spokes. Standard materials for nipples are brass (usually nickel-plated) and aluminum. Brass is heavier but more durable and less prone to corrosion, rounding of flats while tightening, and breaking; aluminum is far lighter. If using aluminum nipples, the spokes should be long enough to engage the full length of spoke nipple threads so that the load is carried from spoke to the top of the nipple. If the spoke is only part way into the nipple, the nipple carries the tension load and is prone to breaking from fatigue failure. Also, with aluminum nipples care and a good spoke wrench are needed to avoid rounding the flats while tightening. Recently, titanium nipples have become available in the US from Titan Wheel Technologies. The theory behind titanium nipples is that they combine the advantages of brass and aluminum nipples (strength and weight, respectively), while also being far more corrosion resistant.
If the underside of the nipple has the traditional conical shape, the nipple tends to adjust perpendicular to the nipple seat of the rim when the spoke is tensioned. This can cause a marked bend in the spoke where it exits the nipple, especially on small wheels with large diameter hubs, and subsequent early spoke failures. Nipples with a spherical contact surface to the rim that do not exhibit this tendency have been introduced (Sapim Polyax, DT ProHead).
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