Arguably the most versatile piece of cycling clothing you can own, gilets keep your core warm, toasty and ultracomfortable. Use them as a single extra layer on your ride to work, or as one of several.
They fold or scrunch down to next to nothing in your commuter bag in case it warms up. Best of all, why not combine them with arm warmers? - surely the most adaptable combination for your commute on all but the coldest days.