The question I get asked most about relaxed fit blazers is whether they'll just look like I've grabbed something off the back of a chair that isn't mine. The honest answer is that they can, but only if you let the size do all the talking. A relaxed blazer is meant to have room built into the cut itself, through the shoulder and the body, not room you've added on top by going up a size because that's what you always do with jackets.
The fit you're after sits a little past your natural shoulder line, not halfway down your arm, and that's the bit people get wrong first. If the shoulder seam is somewhere near your elbow, you haven't found a relaxed fit, you've found a jacket that doesn't fit. The sleeve should stop just below the wrist bone so there's a cuff to push up, because half the point of this shape is being able to shove the sleeves back when you're doing anything useful with your hands. Length is the other thing to watch. Too long and it reads as borrowed, too short and the slouch in the body looks accidental rather than deliberate, so I always try sitting down in one before I decide, since that's when a badly cut relaxed blazer bunches at the back and pulls across the shoulder blades in a way the mirror never shows you standing up. Underneath, keep things close to the body. A relaxed blazer over a baggy jumper or a wide trouser just adds volume to volume, and you end up looking like you're wearing a tent with a smaller tent inside it, which nobody needs.
Where it genuinely falls short is anywhere that wants a sharper line doing the work for you, a formal interview, a wedding where you're meant to look pulled together rather than comfortable, that sort of thing. The soft shoulder and loose body that make it so easy to live in are the same things that won't give you a crisp silhouette no matter how you belt it or roll the sleeves. It's also not kind to a very petite frame unless you shorten the length, because the proportions start wearing you rather than the other way round. What it's brilliant at is everything slightly less formal than that, thrown over a simple top and narrow trouser or a dress, worn open, sleeves pushed up, looking like you decided on it rather than fell into it. That's the one I keep reaching for.
