FIG. 1 is a perspective view of a chair backrest showing my new design;

FIG. 2 is a front elevation view thereof;

FIG. 3 is a rear elevation view thereof;

FIG. 4 is a left side elevation view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a bottom plan view of a chair as environmental structure including the chair backrest showing my new design; and,

FIG. 7 is a cross-sectional view of the chair (as environmental structure) of FIG. 6 taken along the section lines 7—7 illustrated in FIG. 4, to provide a bottom view of the chair backrest showing my new design.

The right side elevation view of the chair backrest is a mirror image of the left side elevation view thereof.

All structures shown in broken lines represent environmental structure and are not part of the design sought to be patented.