In the seventeenth century the organ, the clavichord, and the harpsichord became the chief instruments of the keyboard group，a supremacy they maintained until the piano supplanted them at the end of the eighteenth century.

　　(同位语a supremacy)

　　17世纪时风琴、敲弦琴和拨弦琴成为键盘乐器类的主要成员，这种至高无上的地位一直由它们保持，直到18世纪末被钢琴所取代。

　　分句1：In the seventeenth century the organ, the clavichord, and the harpsichord became the chief instruments of the keyboard group，

　　分句2：a supremacy

　　分句3：they maintained until the piano supplanted them at the end of the eighteenth century

　　分句2是分句1的同位语，用来修饰说明分句1中的the chief instruments of the keyboard group，分句3是省略了 "that" 的定语从句，修饰分句2.