The taus is a bowed instrument shaped as a peacock, with a long fretted neck, gut and steel strings and sympathetic strings under them. Its invention is attributed by tradition to Guru Hargobind Sahib Ji.
The dilruba is the smaller, lighter version, said to have been made for Sikh soldiers who needed something they could carry on horseback. Both are bowed sitars in effect: fretted like a sitar, played with a bow like a sarangi.
They were the standard kirtan instruments before the harmonium and they nearly disappeared in the twentieth century. A small number of makers, principally in Amritsar, kept the craft.
The revival since the 1990s has put them back into some gurdwaras and into the teaching at Punjabi University. Making one takes months and there are very few luthiers.
The saranda, the sarangi and the rabab belong to the same lost set. The Namdharis kept several of these instruments in continuous use when nobody else did, which is why the revival had anything to work from.
The bow is called a gaz, and the playing hand slides along the neck rather than stopping the string at a fret in the sitar manner, so the frets are guides and not stops. That is what lets the instrument bend a note the way a voice bends it, which is the whole reason it was used to accompany singing.
Both were used in gurdwara kirtan with the rabab and the saranda, and all four went out of use as the harmonium spread through the twentieth century.
They are being made and taught again, at a small number of institutions in Punjab and abroad. The number of players is in the hundreds rather than the thousands.