  From a grammar point of view, the role of punctuation marks in a sentence is
formally defined and well understood. In semantic analysis punctuation plays
also a crucial role as a method of avoiding ambiguity of the meaning. A
different situation can be observed in the statistical analyses of language
samples, where the decision on whether the punctuation marks should be
considered or should be neglected is seen rather as arbitrary and at present it
belongs to a researcher's preference. An objective of this work is to shed some
light onto this problem by providing us with an answer to the question whether
the punctuation marks may be treated as ordinary words and whether they should
be included in any analysis of the word co-occurences. We already know from our
previous study (S.~Dro\.zd\.z {\it et al.}, Inf. Sci. 331 (2016) 32-44) that
full stops that determine the length of sentences are the main carrier of
long-range correlations. Now we extend that study and analyze statistical
properties of the most common punctuation marks in a few Indo-European
languages, investigate their frequencies, and locate them accordingly in the
Zipf rank-frequency plots as well as study their role in the word-adjacency
networks. We show that, from a statistical viewpoint, the punctuation marks
reveal properties that are qualitatively similar to the properties of the most
frequent words like articles, conjunctions, pronouns, and prepositions. This
refers to both the Zipfian analysis and the network analysis. By adding the
punctuation marks to the Zipf plots, we also show that these plots that are
normally described by the Zipf-Mandelbrot distribution largely restore the
power-law Zipfian behaviour for the most frequent items.
