Potassium dichromate is the correct and preferred systematic chemical name for the compound also widely known in photographic literature and practice as potassium bichromate, with both names referring to the same bright orange-red crystalline inorganic compound with the chemical formula K2Cr2O7. The prefix di in the systematic name dichromate and the older prefix bi in the historical name bichromate both indicate the presence of two chromium atoms in the dichromate anion, and the two names are completely interchangeable in photographic and chemical contexts.