The Music Stem / The Musician's Library / Composer's Handbook / Invertible Counterpoint
Composer's Handbook
Invertible counterpoint is two lines written so either can be on top.
Design two lines that still work when you swap their registers.
Keep to thirds, sixths and octaves. A fifth becomes a fourth when the lines swap, which sounds unstable.
Bach, The Art of Fugue, Contrapunctus 9