conlangs1-7  Exercise 3: Design Features Across Media
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  Feature            Sign language                    Whistled Spanish
  -----------------  -------------------------------  -----------------------------
  Arbitrariness      Yes, with some resemblance in    Yes. It carries Spanish words,
                     iconic signs                     which are arbitrary.
  Duality            Yes. Hand shape, location and    Yes. Whistles carry the
                     movement are meaningless parts   sounds of Spanish, which are
                     that combine into signs          meaningless parts of words.
  Displacement       Yes. Signers talk about past,    Yes. It carries ordinary
                     future and imaginary things      Spanish, so anything can be
                                                      said
  Productivity       Yes. New sentences are built     Yes, for the same reason
                     freely

What this shows: the medium (hands, face, whistle) does not decide whether
something is a language. The structure does. A sign language has these
features by itself, and whistled Spanish inherits them from Spanish.