FIG. 1 is a front perspective view of a first embodiment of a chair back showing my new design;
FIG. 2 is a rear perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof, a left side elevational view being a mirror image thereof;
FIG. 6 is a top plan view thereof;
FIG. 7 is a front perspective view of a second embodiment of a chair back showing my new design;
FIG. 8 is a rear perspective view thereof;
FIG. 9 is a front elevational view thereof;
FIG. 10 is a rear elevational view thereof;
FIG. 11 is a right side elevational view thereof, a left side elevational view being a mirror image thereof;
FIG. 12 is a top plan view thereof;
FIG. 13 is a front elevational view of a third embodiment of a chair back showing my new design;
FIG. 14 is a rear elevational view thereof;
FIG. 15 is a right side elevational view thereof, a left side elevational view being a mirror image thereof;
FIG. 16 is a top plan view thereof;
FIG. 17 is a front perspective view of a fourth embodiment of a chair back showing my new design;
FIG. 18 is a rear perspective view thereof;
FIG. 19 is a front elevational view thereof;
FIG. 20 is a rear elevational view thereof;
FIG. 21 is a right side elevational view thereof, a left side elevational view being a mirror image thereof; and,
FIG. 22 is a top plan view thereof.
The broken lines showing is included for the purpose of illustrating environment of the design and forms no part of the claimed design.