My beard neckline has crept up under my jaw and I can't work out how to stop doing it

Short beard, tidy the neck with my DE every couple of days. Every single time I do it I take "just a tiny bit more" to even the two sides up, and I've now looked at a photo from June and realised my beard starts basically at my jawbone. It looks like a chinstrap. It did not look like this in June.

I know how it happened, I just don't know how to stop it happening again once it's grown back. Do people actually mark the line somehow, or is everyone else just better at this than me?

Everyone does this. It's the beard version of cutting your own fringe.

What broke the cycle for me was deciding the line once, on a dry beard, in daylight, and then never re-deciding it. The creep comes entirely from redrawing it each time. If you only ever shave up to a line you already committed to, there's nothing to creep.

The evening-up thing has a specific fix: stop working in a mirror at a tilted head angle. When you tip your head back to see under your jaw, the two sides genuinely look uneven because you're looking at them from different angles. So you shave one, then the other, then the first again, and now you're a centimetre up your jaw and it still looks wrong.

Head level, look straight ahead, use a second mirror or your phone camera to check under the jaw rather than tilting. Ninety percent of my creep stopped when I did that.

Also stop tidying every two days. Give it four and the line stops being a daily decision.

The line you want is above the Adam's apple, following the curve of the underside of the jaw, and running back up to just behind each ear. Not the jawline itself. What you've described, a beard starting at the jawbone, is the classic chinstrap and it's the single most common self-inflicted beard problem I fix in the chair.

Two mechanical habits will hold it there once it's grown back:

Shave towards the line from the bare side, never down into the beard. If your hand wobbles going upwards you lose a millimetre of neck you were shaving anyway. If it wobbles going downwards you take a notch out of the beard edge, and then you're evening up the notch, and that's the creep.

And keep your head level with the light in front of you, as robde79 says. I'd add: set the line on a dry, brushed beard, because wet hair lies flatter and sits lower than it will when it dries, so a line that looks right in the shower is usually too high by the time you're dressed.

A safety razor is a good tool for clearing the neck cleanly, but it isn't an edging tool, so don't expect a knife-sharp architectural line from it. If you want that, it's a shavette or a barber, and then your job at home is only maintaining what was set. The whole approach, cheek line and under the nose included, is in shaving around a beard or moustache.

Last thing, and it's the one people ignore: the strip of neck right under a beard line is the most irritation-prone skin on the face, and tidying it daily to keep a perfect edge is a reliable way to get razor burn and, for curly hair, ingrown hairs. A slightly soft edge you shave every third day beats a crisp one that's permanently red. If it stays sore for weeks or looks infected, that's a pharmacist or doctor, not a technique tweak.

Photo trick that helped me: take a picture of the line the day it looks right, head level, and keep it on your phone. Comparing against a photo is brutally more honest than comparing against your memory of what it looked like. I found out mine had moved about a centimetre over two months and I would have sworn it hadn't moved at all.

Three weeks of growing it back out and I've now set the line properly, dry beard, head level, phone photo saved. Tidying every third day instead of every other day, shaving upwards to the line only.

Hasn't moved yet. Neck is also much happier for the extra day between tidies, which I hadn't expected to be part of the answer but apparently it was the biggest part.