Brahms had already composed over 20 string quartets (he confessed to a friend that he had burnt all of that stuff) before he finally presented his Opus 51 to the public. In the end only three surviving works managed to withstand his high self-criticism; numerous layers of corrections bear witness to his struggle with each detail. We are now publishing the two passionate dark Quartets op. 51 separately from the rather more lively Opus 67. The basis for this edition is the volume in the New Complete Edition of Brahms' works, published in 2004 by Henle, for which previously missing sources in a Swiss bequest were consulted for the first time.