The string trio as an independent genre developed in the 18th century - based on the trio sonata for two solo voices and an independent, sometimes contrapuntal bass voice. Around 1750, the string trio genre was from much greater significance as the string quartet, which was still in its early stages. The specificity of the string trio is not limited to the presence of the three instruments; much more important is their use in a new compositional structure. While the Baroque trio tradition was based on two equal upper voices with a supporting bass voice, the string trio cultivates a structure in which all three instruments tend to be of equal value. A first highlight of the new genre, after contributions by L. Boccherini, J. Haydn and others, was reached with W. A. Mozart's Divertimento in E-flat major, K. 563 (1788). The title (Divertimento, similarly also: Serenade) still hints at - for the string trio constitutive - traditions of ballroom music, in which creative freedom often became effective in a playful way. In terms of content, the trio formation has already distanced itself far from this tradition and shows itself to be chamber music of the highest order.
L. van Beethoven (op. 3 and Serenade op. 8) also initially oriented himself on the traditional sequences of the divertimento; his three trios op. 9, however, are already arranged as large-scale sonata cycles and mark a new genre claim. The mere fact that Beethoven contributed five important works to this genre shows their importance in the music of this time. The 19th century, which strove for tonal opulence, gave preference to the string quartet as a genre, repressing the string trio from its primacy in the 18th century.
It was not until the early 20th century that the genre experienced a renaissance, including the trios of Max Reger among others. With its three-part harmony reduced to the essentials and the "absence" of a doubling voice (as practiced in a quartet), the string trio offered every player attractive, equal opportunities. These were used right up to the present within the framework of rich stylistic orientations and highly individual creative concepts.