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Variant forms of characters have existed since the days of oracle bone script. It wasn’t until the publication of the first character dictionary, the Shuōwén Jiězì [說文解字] (often referred to as simply “the Shuōwén“) in 100 CE that the notion of a standard form for a character began to emerge, the Shuōwén itself acting as a de facto standard. Before that, there were common ways of writing certain characters, but there was a lot of variation too; so much in fact, that it can be shocking for those who are used to the highly standardized modern script. Having said that, there many variants within the modern script as well. See the Taiwan Ministry of Education’s Dictionary of Chinese Character Variants, where you can look up variants for any character of interest.