In the early 1960s, the Nashville sound began to be challenged by the rival Bakersfield sound on the country side and by the British Invasion on the pop side; compounding these problems were the sudden deaths, in separate airplane crashes, of Patsy Cline and Jim Reeves, two of the Nashville Sound's biggest stars. Nashville's pop song structure became more pronounced and it morphed into what was called Countrypolitan-a smoother sound typified through the use of lush string arrangements with a real orchestra and often, background vocals provided by a choir. Countrypolitan was aimed straight at mainstream markets and it sold well throughout the later 1960s into the early 1970s. Among the architects of this sound were producers Billy Sherrill (who was instrumental in shaping Tammy Wynette's early career) and Glenn Sutton. Artists who typified the countrypolitan sound initially included Wynette, Glen Campbell (who recorded in Hollywood and not Nashville), Lynn Anderson, Charlie Rich, and Charley Pride, the latter being a rare example of a top-selling African-American country performer.

Based on this passage, the nashville sound brought a polished and cosmopolitan sound to country music by? Let's think fast. Stream of consciousness:
The relevant information to answer the above question is: Nashville's pop song structure became more pronounced and it morphed into what was called Countrypolitan-a smoother sound typified through the use of lush string arrangements with a real orchestra and often, background vocals provided by a choir. So the final answer is the use of lush string arrangements with a real orchestra and often , background vocals provided by a choir.