Etching of synthetic diamond single crystals in NaNO3 and KNO3 melts at 750-800 degrees C has been carried out to study linear etch figures and related etch pits. Two types of planar defects in I I I I I planes in large synthetic diamond crystals have been found from the structure of linear etch figures. They are microtwin lamellae and stacking faults. It is established that linear etch figures, bound by the etch pits with alpha(1 1 1) = 3 degrees are formed at the emergence lines of stacking faults. The linear etch figures, bound by the etch pits with (alpha)(1 1 1) = 2 degrees or showing no etch pits are formed at the emergence lines of the microtwin lamellae or double stacking faults. Possible types of partial dislocations bounding stacking faults and triangular pits bounding the linear etch figures have been correlated. The analysis has shown that shallow etch pits with inclination angles (alpha)(1 1 1) = 2 degrees and 3 degrees are formed at the emergence points of partial dislocations and etch pits with alpha(1 1 1) = 4 degrees at the emergence points of screw dislocations. (c) 2007 Elsevier B.V. All rights reserved.