The ornamental design for a back and seat for a chair, as shown and described.
FIG. 1 is a perspective view showing the left side, top and front of a back and seat for a chair which forms my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a left side elevational. view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The longer length broken lines shown in the figures illustrate environment that forms no part of the claimed design. The shorter length broken lines shown in the figures illustrate stitching that forms part of the claimed design.