The thing nobody tells you about a square neck is what happens to your bra straps. Weeks two and three in, you realise you've stopped thinking about them altogether, because the neckline sits straight across the collarbone and actually covers a normal bra rather than forcing you into a strapless one that slides down by lunchtime. That's the whole appeal, really. It just took me longer to notice than it should have.
This shape earns its spot across most of the year rather than one specific season. It works under a jacket for a lunch that's nominally about work but isn't really, it works bare armed in August with sandals, and it works with tights and boots the second it gets cold, because the neckline reads a bit more considered than a round or scoop neck so the rest of the outfit can be completely plain. I've worn the same one with white trainers to walk a dog and with heeled mules to something that needed slightly more effort, and nobody clocked it was the same dress because the neckline did a different job each time depending on what sat under it, a chain, nothing, a scarf tied flat across the chest.
If you've got none of these, start with a midi in a fabric with a bit of structure rather than a floaty jersey. A square neck relies on some stiffness to keep its shape away from the body, and a soft jersey one tends to fold in on itself and lose the line within about an hour of sitting down. A cotton poplin or a firmer linen blend holds the shoulders square the way it's meant to.
The difference between a good one and a sad one is almost always in the facing, that strip of fabric finishing the inside edge of the neckline. On a well made dress it's cut properly and sits flat, no puckering, no visible stitching pulling the corners in. On a cheap one it curls after a few washes and the corners of the square start to round off, which defeats the entire point of buying the shape in the first place.
In the shop, check the corners sit flat against your collarbone without digging in, check the straps are wide enough not to slip if they're not built in one piece with the bodice, and sit down in it before you decide, because a neckline that looks sharp standing up can gape completely different once you're folded into a chair.