Main idea

An instruction along the lines of "keep it short" doesn't long survive the urge to put everything in: you add the source, the nuance, the development, and the card becomes a mini reading note. The rule is respected when it's easy, worked around when it gets in the way.

A physical format isn't negotiable. A card close to A7 holds a title and three to five lines; beyond that, there is no room, and the trade-off is made on the paper, at the moment of writing, with no discipline to summon.

The choice of brand, for its part, is a supply question rather than a methodological one: Oxford cards and Moleskine notebooks are chosen because you'll plausibly still be able to buy them in several years. What matters is that every card be the same size, with distribution durable enough for the format to last as long as the practice.

Why it matters

It moves the effort from willpower to the setup: a constraint built into the object stops being a decision to make again every day.

It also gives a rarely stated material criterion: a format you won't be able to buy in five years weakens a practice that assumes its cards are uniform.

Nuances and limits

The constraint can cut in the wrong place: an idea that deserved eight lines loses two, and the ellipsis sometimes makes the card less evocative than slightly longer text.

Open questions

Would a digital equivalent — a field capped at five lines — produce the same trade-off, or does the constraint only work on a support you can't stretch?