sentences:  Numerous "S" models, from most of Audi's mainstream model ranges have been produced over the years.These cars are recognisable by their "S" badges, and unique emphasis lines on their front grills and on the back side, instead of the letter "A" in the designation of numbers and also in use in the notation "S3", "TTS" and "SQ5".All Audi "S" models are equipped with Audi's 'trademark' quattro four-wheel drive system as standard.