Page "Graphics Interchange Format" Paragraph 198
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Conversely, the symbol width could be set at 8, even if only values 0 and 1 are used ; these data would only require a 2-color table.
Although there would be no point in encoding the file that way, something similar typically happens for bi-color images: the minimum symbol width is 2, even if only values 0 and 1 are used.
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