Quindicesima (15)
A plain 15 begins a two-octave displacement line. Above the staff it raises the covered passage two octaves; below it lowers it, unless an explicit alta or bassa form removes the ambiguity.
- Octave signs
- Above the staff
- Numbers
Do not confuse it with
A plain 8 shifts one octave; a plain 15 shifts two.
What the sources say
- SMuFL — Octaves
15 (quindicesima) ... may be used to indicate the raising or lowering of pitch by ... two ... octaves
- Octave lines
15ma alta (2 octaves above) and 15ma bassa (2 octaves below) are also occasionally used.
- Steps to Music Theory — The Basics
Quindicesima: 15ma is also written below the notes ... to indicate which notes sound two octaves lower than written.