One of many definitions of ''layering'' dictionary.com - the wearing of lightweight or unconstructed garments one upon theother, as to create a fashionable ensemble or to provide warmth without undue bulkiness or heaviness.
Autumn gives you so much space for improvisation. With relatively cold mornings and warmer afternoons, a turtleneck will never fail you as a replacement for a larger scarf which would give you more to carry on hectic days. With a grey coat and a dark green blazer (one of this season's most popular colors), you can easily tone down the overall feel with a turtleneck. Layering is many ways a more stylish alternative to just throwing on a huge jacket.