  Zipf's law is found when the vocabulary of long written texts is ranked
according to the frequency of word occurrences, establishing a power-law decay
for the frequency vs rank relation. This law is a robust statistical property
observed even in ancient untranslated languages. Interestingly, this law seems
to be also manifested in music records when several metrics---functioning as
words in written texts---are used. Even though music can be regarded as a
language, finding an accurate equivalent of the concept of words in music is
difficult because it lacks a functional semantic. This raises the question of
which is the appropriate choice of Zipfian units in music, which is extensive
to other contexts where this law can emerge. In particular, this is still an
open question in written texts, where several alternatives have been proposed
as Zipfian units besides the canonical use of words. Seeking to validate a
natural election of Zipfian units in music, in this work we find that Zipf's
law emerges when a combination of chords and notes are chosen as Zipfian units.
Our results are grounded on a consistent analysis of the statistical properties
of music and texts, complemented with theoretical considerations that combine
different reference models, including a simple model inspired in the Lempel-Ziv
compression algorithm that we have devised to explain the emergence of Zipf's
law as the consequence of languages evolving into more efficient forms of
communication.
