Human tooth enamel is formed by grains which, when observed with the electron transmission electron microscope, sometimes exhibit a line 1 to 1.5nm thick along their centers. This line has been called “dark line.” 1–3 A distinctive feature of the line is that it appears dark in defocus, disappears when the image goes through focus, and is white bordered by two slightly darker fringes in overfocus. This line is observed in conjunction with the (100) lattice fringes of the hydroxyapatite when the grains are observed along the [010] zone axis and parallel to one of the (110) planes when the grains are observed along the [001] zone axis. 4 The occurrence of this line is of particular interest because it seems to go under preferential dissolution during early stages of caries. 5 However many questions remains unanswered as to what the central defect plane really is and what its role in the enamel structure is. In this work we present the results obtained on the human tooth enamel grains and on the dark line when techniques of transmission electron microscopy, x-ray diffraction, digital processing and image simulations are used.