The Guru Granth Sahib Ji is arranged by raga. Thirty-one ragas structure the volume and the instruction for how a composition is to be sung is printed above it, sometimes down to the tune of a named folk vaar.
That makes it a musical document as much as a textual one. Singing a shabad in the raga assigned to it is not decoration; the raga was chosen for the composition and carries part of its meaning.
Guru Nanak Dev Ji travelled with a rababi. Guru Arjan Dev Ji employed court musicians. Guru Hargobind Sahib Ji had dhadis. The Sikh tradition has never separated the text from its performance.
Through the twentieth century the raga discipline weakened in ordinary gurdwara practice, displaced by harmonium-led singing in a small set of light tunes. The harmonium itself arrived with the British in the nineteenth century and is now what most people picture.
Since the 1990s a revival has worked to restore raga-based kirtan and the stringed instruments, taught at Punjabi University's gurmat sangeet department and by a number of ustads. It has changed practice at some institutions and not at most.
The Guru Granth Sahib Ji is organised by raga, thirty-one of them, and the instruction to sing a hymn in its raga is part of the text.
The classical performance tradition was carried by the rababi families and it went in 1947, and what replaced it was harmonium led kirtan in a lighter idiom.
Gurmat sangeet departments now exist at the Punjabi universities and a revival of the ragas and the string instruments has been under way for about two decades.