Butterfly handlebars are a fairly flat highly curved handlebar that support a large number of hand positions while the cyclist maintains an upright posture. Butterfly bars are sometimes known as trekking or "euro" handlebars because of their popularity with European touring cyclists.

Butterfly bars come in a number of different shapes, but are characterized by being largely flat, and composed of two rounded "wings" that include both front and back hand positions (perpendicular to the main axis of the bike) as well as positions parallel to the main axis of the bike. This parallel position is especially well suited to climbing with a load, as it provides significant leverage for driving the pedals.

Unlike drop handlebars, butterfly bars leave the rider in an upright riding position. To achieve a more aerodynamic position with butterfly handlebars, rather than shifting your position down as you would with drop bars, you move your hands forward, stretching and slightly lowering your posture.

Butterfly bars typically offer four or five primary hand positions, with varied widths and reach. This diversity of hand positions helps make the butterfly handlebar well suited for long distance cycling, since you can shift your hand position and posture to avoid stiffness.

Butterfly bars are sometimes confused with mustache handlebars, and they do look similar. Unlike mustache bars, which butterfly handlebars typically include a "flat bar" hand position (perpendicular to the main access of the bike) closer to the rider than the stem is. (This is the segment marked E in the diagram.) Mustache handlebars do not include this segment, and typically have a bit more drop than trekking bars.

You can learn more about bicycle handlebars in Understanding Handlebars.