Crystallographic twins are critical to the properties of numerous materials from magnesium alloys to piezoelectrics. Since the onset of the twin formation is highly sensitive to the triaxial mechanical boundary conditions, non-destructive bulk microscopy techniques are required. Elastic strains can be mapped via X-ray diffraction with a 10 0-20 0 nm resolution. However, the interplay of strains with nanotwins cannot be characterized. Here, a method based on dark-field X-ray microscopy to quantify the density of nanotwin variants with twin lamellae of sizes as small as several tens of nanometers in embedded subvolumes (70x20 0x60 0 nm 3 ) in millimeter-sized samples is introduced. The methodology is corroborated by correlating the local density of twin variants to the long-ranging strain fields for a high-performance piezoelectric material. The method facilitates direct, in situ mapping and quantification of nanoscale structural changes together with their elastic driving fields, which is the key towards controlling and engineering material?s performance at nanometric scales. ? 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )