Deformation twins related to two separate (111) orientations have been observed in a copper single crystal deformed by tensile extension at 4.2°K. The unambiguous determination of the twin relationships is accomplished by a unique x-ray technique which makes use of a single crystal adapter and Geiger counter methods. The adapter permits the detection and measurement of weakly diffracting reflections and the twin relationships are established by the unique application of geometrical data in a stereographic projection.