  The syntactic structure of a sentence can be represented as a graph where
vertices are words and edges indicate syntactic dependencies between them. In
this setting, the distance between two syntactically linked words can be
defined as the difference between their positions. Here we want to contribute
to the characterization of the actual distribution of syntactic dependency
distances, and unveil its relationship with short-term memory limitations. We
propose a new double-exponential model in which decay in probability is allowed
to change after a break-point. This transition could mirror the transition from
the processing of words chunks to higher-level structures. We find that a
two-regime model -- where the first regime follows either an exponential or a
power-law decay -- is the most likely one in all 20 languages we considered,
independently of sentence length and annotation style. Moreover, the
break-point is fairly stable across languages and averages values of 4-5 words,
suggesting that the amount of words that can be simultaneously processed
abstracts from the specific language to a high degree. Finally, we give an
account of the relation between the best estimated model and the closeness of
syntactic dependencies, as measured by a recently introduced optimality score.
