Solfège syllables were created by Guido d'Arezzo (990–1050), an Italian monk, and used with his innovative 4-line staff notation system. These syllables are especially valuable for singing since they naturally fit the human voice. Hand signs were created by Englishman Rev. John Spencer Curwen (1816–1880) to help his choir to sing on pitch. Hand signs give a physical placement for vocal pitches that helps with memory and singing on pitch.
It is best to use both hands for brain coordination and body balance. Both sides of the brain are active. Using both hands also helps students model teachers without left-hand or right-hand confusion.
Some teachers use sol and si in place of so and ti.
Drawing adapted from www.classicsforkids.com