The term sackbut refers to the early forms of the trombone commonly used during the Renaissance and Baroque eras. A sackbut has the characteristic telescopic slide of a trombone, used to vary the length of the tube to change pitch, but is distinct from later trombones by its smaller, more cylindrically-proportioned bore, and its less-flared bell. Unlike the earlier slide trumpet from which it evolved, the sackbut possesses a U-shaped slide with two parallel sliding tubes, rather than just one.

Records of the term trombone predate the term sackbut by two decades, and evidence for the German term Posaune is even older. Sackbut, originally a French term, was used in England until the instrument fell into disuse in the eighteenth century; when it returned, the Italian term trombone became dominant. In modern English, an older trombone or a replica is called a sackbut.

The bell section was more resonant, since it did not contain the tuning slide and was loosely stayed rather than firmly braced to itself. This trait and its smaller bore and bell produce a "covered, blended sound which was a timbre particularly effective for working with voices,... zincks and crumhorns", as in an alta cappella.

The revived instrument had changed in specific ways. In the mid-18th century, the bell flare increased, crooks fell out of use, and flat, removable stays were replaced by tubular braces. The new shape produced a stronger sound, suitable to open-air performance in the marching bands where trombones became popular again in the 19th century. Before the early 19th century, most trombones adjusted tuning with a crook on the joint between the bell and slide or, more rarely, between the mouthpiece and the slide, rather than the modern tuning slide on the bell curve, whose cylindrical sections prevent the instrument from flaring smoothly through this section. Older trombones also generally don't have water keys, stockings, a leadpipe, or a slide lock, but as these parts are not critical to sound, replicas may include them. Bore size remained variable, as it still is today.