If you're packing for a trip and you know you'll need dress shirts, a suit or a jumper, save yourself time and energy and pack plain T-shirts you can wear alone instead of singlets. They can serve the under-shirt purpose, and you'll have extra clothing options if the weather turns out warmer than you expect.
Photo by Brianna Laugher
It's an old packing trick, but still a good one to remember. If you know you'll need to wear heavy clothes at some times and lighter clothes other times, packing plain T-shirts is a good idea. Then, you can save space and not pack singlets, and wear the T-shirts under your dress shirts or sweaters instead. Plain white is your friend here!