Callahan V Back Sweater in Black. - size XS (also in M) Callahan V Back Sweater in Black. - size XS (also in M) 100% cotton. Hand wash cold. Knit fabric. CAHN-WK24. 8166. Los Angeles based brand Callahan is known for their comfy sweaters and perfect boyfriend fits. Their line is elevated by unique textures, materials, and techniques that give each piece versatility and wearability. Designed for cool girls everywhere, Callahan is inspired by effortlessly easy style.