Transmission electron microscopy (TEM), high-resolution X-ray diffraction, and KOH etching have been used to study the dislocation structure of 4H SiC crystals grown by the physical vapor transport method. Many of the etch pits on the Si(0 0 0 1) surface form arrays extending along the [(1) over bar 1 0 0] directions. Plan view conventional and high-resolution TEM show that the arrays consist of pure edge dislocations threading along the c-axis with identical Burgers vectors of the a/3[1 1 (2) over bar 0] type. The dislocation arrays constitute low angle [0 0 0 1] tilt boundaries, i.e., [0 0 0 1] is the common axis lying in the boundary. Typical values of the misorientation are in the 60-200 arcsec range. Evidence is presented that such boundaries can form by polygonization of the threading edge dislocations, which have been introduced into SiC crystals by prismatic slip.