Brahms’ Horn trio has the cello in a piano trio replaced by a French horn. It was written specifically for a natural horn, without valves. Brahms was aware of the recent developments in horn technique, where the valves made the reach of the instrument wider by lengthening the path of the air. By pressing a valve, the length of the tube is lengthened. A modern double horn allows the player to use twelve keys that way. But Brahms composed it for the earlier, valveless version for some reason.
The trio was the first work Brahms completed after the death of his mother. His parents were quite different in age, and that created tension between the couple. His mother Christiane was twelve years senior. Though Brahms really tried to keep the two together, in 1864 he had to concede and help his father to another a different place to live. In letters he tried to make sure they still cared about each other. It was to no avail. Two years later his mother died.
What is left to us is this relatively short work. It has been suggested to Brahms he could replace the horn with a cello, but he didn’t like the sound of that. Instead he wrote an alternative for viola some years later.