The notebook for lefties has slanted lines so that the writer doesn't smudge their writing.
Now, this whole concept feels pointless to me. And before you all lefties weigh in and tell me how horrendous it is to be left-handed in a right-handed world, I should point out that I'm ambidextrous. I started writing with my left hand, was coerced into writing with my right at my first primary school, generally write with my right hand but for >2 years of my PhD wrote only with my left because I had crippling RSI in my right wrist. I can still write left-handed and frequently did when I taught, because it meant I could stand on either side of the white-board and not block anyone's view while writing. I have never smudged a single line of writing, whether writing with my left or with my right hand. At least not through the actual act of writing! I may well have forgotten that some inks take longer to dry than I expected and ended up smudging things, but never just through the act of writing. Never.
I hold the pen in exactly the same way, whether I hold it in my left or my right - it looks like a mirror image. The nib of the pen or pencil is about 3-4 lines away from the rest of my hand, I'm resting on the smallest finger and part of the ring finger, and all of my fingers are closer to me than the nib of the pen/point of the pencil. In fact, I hold a pen in my left hand pretty much the same way as the leftie in their video does only with a slightly more closed hand:
I don't see how anyone holding the pen like that will have a smudging problem...
As a quick test: right-handers out there - draw some scribbles with a pen (like loopy writing) but write from right to left across the page. Did you smudge it with the rest of your hand? I'm betting you probably didn't, any more than I do when writing left to right with my left hand.
So why do so many lefties complain about smudging their writing? Do lefties hold their pens in weird ways? If so, why? Because they needed to see the letters they'd written when they were learning to write? But, I can see all the letters as I write with my left.
I know many lefties do find it an issue, but I'm genuinely perplexed by this. Could the left-handed readers please let me know why their writing smudges?
[Incidentally, I totally get why the rings in a Filofax make writing on the right-hand page uncomfortable, because that is the same for right-handers writing on the left page!]