These TEM (Transmission Electron Micrographs) pictures must be treated with caution. If the experimentalists take the same sample and look at it with a different imaging method, they can make the widths and lengths of the diagonal stripes seem to change. We don't know how to take these pictures and make quantitative measurements of the tweed pattern. Also, the electrons pass through a thin slice of material (transmission electron): it's always possible that the tweed is different in a thin slice than in the bulk, and some people have proposed that the cross-hatched patterns are superpositions of simple stripes which change from one direction to the other in the thickness of the material. The TEM pictures were how tweed was discovered, and remain the most vivid illustrations.
James P. Sethna, firstname.lastname@example.org
Statistical Mechanics: Entropy, Order Parameters, and Complexity, now available at Oxford University Press (USA, Europe).