2 consecutive gracenotes; The 1st has the same pitch as the target note, the other is a second higher; Made with a pat of the finger commanding the note above the sounding one, immediately after the latter commenced:

g¹-a¹ - g¹; a¹-b¹ - a¹; b¹-c² - b¹; f♯²-g² - f♯²; g²-a² - g²; a²-b² - a²; Rare in descending melody lines; Doubles with a lowered second are not used.
NB: Example(s) to be replaced with staff-notation.