Brahms had already composed more than twenty string quartets (he will confess to a friend having burned all these "stuff") before daring to present his opus 51. Of this work, only three works have remained, having escaped to the ruthless self-censorship of the composer. We publish both op. 51, passionate and dark, separated from an op. 67 rather cheerful, about which contemporary criticism wrote: "Brahms seems to have this time resolved to walk through a sunny meadow." Our edition is based on the volume, published in 2004 by Henle Editions, of the Neue Brahms-Gesamtausgabe (a new complete edition of Brahms' works), a volume for which, for the first time, the edition was able to rely on sources , previously extinct, from a Swiss legacy.