A homogeneous strain involving either plane strain (one of the principal strains is zero) or general strain, in which material lines that are parallel to the principal axes have the same orientation before and after deformation (non-rotation of principal axes)is called pure shear.
In progressive pure shear, the principal strains do not change their orientations with the progress of deformation. The upper part of the figure shows the fate of 4 differently oriented lines during progressive pure shear. The red line always or at all stages of deformation falls in the contractional area of the strain ellipse and is being shortened all the time, the geologically realistic situation being CONTINUOUS FOLDING OR BUCKING. The blue line, on the other hand falls at all stages of deformation in the extensional area of the strain ellipse and is therefore continuously extended or increased in length, the geologically realistic situation being CONTINUOUS BOUDINAGE. The purple line first undergoes shortening of length and then its length is restored to the initial length and then its length gets increased. The geologically realistic situation is FOLDING, UNFOLDING and then BOUDINAGE giving rise to BOUDINAGED FOLDS. Finally, the green line does not change in length at the first increment of deformation since it lies parallel to the line of no finite longitudinal strain but later it increases in length, the geological situation being boudinage with development of boudins that show en echelon arrangement.
The lower part of the figure shows progressive homogeneous simple shear. The red line is continuously shortened in length and shows ACTIVE BUCKLING since it always or at all stages of deformation by simple shear lies in the negative extensional field. The green line on the other hand, always lies in the field of positive extension and is ACTIVELY BOUDINAGED.
Note that the width of the zone of deformation in the lower figure is constant and all lines parallel to the direction of shear are unchanged in length. These planes are therefore lines of e=0 or 1+e=1 or the lines of no finite longitudinal strain at all stages of deformation by simple shear.