Writing systems were preceded by proto-writing, which used pictograms, ideograms and other mnemonic symbols. Among the symbols used, there are cave paintings, petroglyphs, pictograms and ideograms. These symbols may also be referred to as ideograms or ideographs. Some systems also use ideograms, symbols denoting abstract concepts. These ideograms or "signifying" signs symbolize objects or commodities. Pictograms, in turn, evolved into ideograms, graphical symbols that represent an idea. It is a ceramic vessel bearing inscribed ideograms of celestial objects. Using variation selectors, it is possible to specify certain variant CJK ideograms within Unicode. These Manichaean manuscripts contain no ideograms. Clavijero never ceased to try to read the ideograms in the codices. Unicode (as of version 5.0) encodes 123 Linear B ideograms. In Shanghai, Bliss became interested in Chinese characters, which he mistakenly thought were ideograms. Because some ideas are universal, many different cultures developed similar ideograms. Examples include tongs, spatulas, special moulds to create ideograms with incense powder, etc. There is sometimes a surname to supplement or replace the ideograms.