Tonic sol-fa is a pedagogical technique for teaching sight-singing, invented by Sarah Ann Glover of Norwich, England and popularised by John Curwen who adapted it from a number of earlier musical systems. It uses a system of musical notation based on movable do solfège, whereby every tone is given a name according to its relationship with other tones in the key: the usual staff notation is replaced with anglicized solfège syllables or their abbreviations. "Do" is chosen to be the tonic of whatever key is being used. The original Solfege sequence started with "Ut" which later became "Do".
By the end of the nineteenth century, this notation was very widespread in Britain, and it became standard practice to sell sheet music with the tonic sol-fa notation included.