Two types of ambiguity concerning indexing of electron diffraction patterns have been cleared up by making use of the Kikuchi lines. A convenient unambiguous method of uniquely indexing patterns is suggested assuming the beam is always along the viewing direction no matter how the picture is printed. It is shown that there are two completely equivalent ways of indexing the Kikuchi pattern, e.g., [111] or [1¯1¯1¯]. The method is also useful in interpreting dark‐field pictures when top‐bottom contrast is concerned.