Cards in a deck don't like to lie crooked: if you turn one slightly, it will seek a position where its pattern aligns with the neighbor. Atoms in crystals behave the same way. When two thick crystals of MoS2 or MoS2 with WS2 are stacked with a twist of just 1–2 degrees, their atomic lattices begin to adjust to each other, forming regions of perfect stacking.

Previously, such rearrangement was only seen on surfaces or in thin films, because there was no way to look inside a thick crystal. Now scientists have applied multilayer electron ptychography: an electron beam scans the sample, and the structure at different depths is reconstructed from the scattering. It's like an X-ray for a deck of cards: you can see how each layer inside has shifted.

It turned out that at the interface between two MoS2 crystals twisted by 1.23 degrees, atoms shifted more than calculations for thin films predicted. The outer layers of a thick crystal hinder the inner ones from bending, so they compress and stretch more strongly in the plane. The rearrangement penetrates several layers deep but quickly fades. Now that hidden interfaces have become visible, we can design materials with desired properties, just like selecting cards so they fit perfectly in the deck.