Silk decorations in webs of orb-weaving spiders are considered visual signals. However, high pattern variation reduces the plausibility of a single signal function, and accordingly, current literature often presents contradicting explanations. The controversial question is whether specific web decoration shapes also serve specific functions or whether various arrangements can serve a single function. I studied inherent characteristics of the variation of silk decoration shapes in Argiope bruennichi (Scopoli, 1772), in the field and under invariant laboratory conditions. The results show, that even within a few days and with a small repertoire of patterns, A. bruennichi frequently varies the decoration shape. Field and laboratory data both reveal that this variation follows a predictable pattern, significantly differing from random. Spiders show a preference for two-banded decorations, omitting decorating altogether rather than building one-banded decorations. Since the shape matters to the spiders, this supports the idea of a single signal function in spite of the presence of web decoration pattern variability.