Dressing for the elements in winter is easy: No matter how rushed you are, you know you need to grab your top coat on the way out.
But in the crisp autumn months before an overcoat is necessary, things get a bit trickier. You check the weather report, even step outside, then go back in and wonder if a sportcoat will be enough to keep you warm, and if an under shirt might help.
But there are general guidelines that clothes-wearing men follow when it comes to adding layers, and they basically come down to keeping the level of formality consistent.
Because a suit is more formal, it requires a more formal layer underneath. A patterned vest is a natty option, but its fabric should harmonize with the suit. A tweed vest would pair better with a flannel suit and button cuff shirt than it would with a sober charcoal worsted and spread collar shirt with French cuffs. Keep your country and city clothes separate, gentlemen.
To add a knit, a cardigan vest is another good option with a suit because it mimics a traditional suit vest. A sleeved cardigan, though, is a bit too cozy-looking to pair with a business suit, and is better with a sportcoat.
As for turtlenecks, they look great under a sportcoat, and daring dressers can even pull them off with a suit.