For you newer golfers, there was an amazing professional named Sam Snead who played in the era before Jack Nicklaus and Arnold Palmer. He was a contemporary of Ben Hogan ( of whom I hope you've heard ) and is the all time record holder with 82 PGA Tour wins. He was so good, that the legendary teacher Harvey Penick actually quit competitive golf after seeing Snead hit balls, believing that if he had to compete against the likes of Snead he would make a better teacher than player.
Snead's swing was very fluid and smooth and amazingly good technically. He made a huge turn on the back swing and swung his left arm right along his shoulder plane. His shoulders also turned on a very steep angle and allowed his right elbow to go behind his body, not in front like Nicklaus.
His downswing was nearly perfect, as he keep his posture and rotated through the shot. Notice in the above photo that at impact his right arm is pointing directly down the shaft and at the ball. This type of position, along with the huge body turn back and through, are great mechanics for a rounded golf swing.
An upright player will have his left arm ( or lead arm ) pointed at the ball at impact because they stand tall at address and swing th club more up and down than around. Players who know if their swing is rounded or upright can use this information to help them stay on track and hitting consistent shots.