What's a M.A.C #138 Brush?
A domed brush of ultra-soft, natural fibres featuring a slightly pointed tip with round chiselled sides. Specially designed to streamline and enhance the makeup technique of sculpting and shaping. Use chiselled side for contour shaping of facial planes; the pointed tip for applying and blending powder blush, highlighter or face powder.

(using google images I created a montage. *Thanks Google*)
M·A·C 138 Tapered Face Brush - $52
Dupe: Royal Silk C130 Pointed Dome Powder Brush - $18.75
For a more affordable version of the MAC #138 Face Brush go here:
Naimies.com