In crystallography, the phrase 'perfect crystal' can be used to mean "no linear or planar imperfections", as it is difficult to measure small quantities of point imperfections in an otherwise defect-free crystal.
in crystallographY, tHE pHrasE 'pErfect CrYStal' cAn bE Used To mean "NO lInEar Or pLANar ImperfectioNs", aS iT is dIfficuLt TO measURe sMALL quanTiTies Of poinT iMperfEctionS in an OTherwIse defEct-fREE crysTaL.