Two coats can share the same wool, the same length and the same colour and still look like different garments. The difference is the front. A single-breasted coat has one row of buttons and the two fronts barely overlap. A double-breasted coat has two rows, and one front crosses well over the other to reach the far row.
That small structural choice decides how much of the coat reads as a line and how much reads as a panel. Try it below: switch the front, close or open the coat, and swap the lapel.
Think of the coat as a surface with vertical lines on it. A single row of buttons gives one line down the middle. Two rows give two lines and a broad panel between them, which adds weight to the upper half of the outfit and makes the whole coat look more tailored.
The double-breasted front also changes the lapels. They are usually wider, and many have peak lapels, where the edge points up towards the shoulder instead of stepping in a notch. Single-breasted coats more often have notch lapels, which sit flatter and look more casual.
One row is a line. Two rows are a statement.
Choose the single-breasted front when the coat is the quiet layer: over jeans and a knit, over a suit, over anything you already wear every day. It is easy to open and close, and it never competes with what is underneath.
Choose the double-breasted front when you want the coat to be the outfit. It carries a plain base of one colour on its own, such as a dark knit and straight trousers, and it handles a dressier evening without needing a jacket under it.
Wear it closed for the full effect. If you want it open, keep what is underneath simple, because the overlap is already a lot of fabric to look at.
Whatever the budget, check four things. First, that the overlap lies flat and the fronts do not pull or gape when buttoned. Second, that the buttons are sewn on firmly and match the coat's weight. Third, that the lapels roll cleanly instead of curling. Fourth, that the fabric is dense enough to hold a crisp edge down the front.