Beginning in the latter half of the 20th century, artifacts bearing markings dating to the Neolithic period have been unearthed at several archeological sites in China, mostly in the Yellow River valley. These symbols have been compared to the oracle bone script—the earliest known forms of Chinese characters, first attested c. 1200 BCE—and have been cited by some as evidence that Chinese writing has existed in some form for over six millennia. However, the Neolithic symbols have only been found in small numbers, and do not appear to go beyond pictorial techniques, as is required to obtain a true writing system representing spoken language.