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I have read that a plane should be suitable for flattening a board about 2–2.5 times its length.

I have also read (in several places) that with skill/care/attention one can extend this ratio. But what exactly is meant by that? I don’t recall ever seeing an explanation.

To be clear, I am mainly interested in planing faces in decent boards. For edges, I can cheat a little and prepare a reasonably straight edge with a track saw before taking a very thin shaving.

As a practical example, suppose I have a board that’s 200×32 cm, and my largest plane is 40 cm long (5× ratio). Even if I had a jointer/try plane, that would be about 3.5× at best. Concretely, what would I need to do differently, compared to a smaller board?

P.S. I am happy to accept that for this size, “if it looks flat, it’s flat”! Just curious if there are any specific techniques people are referring to but not naming.

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    This has been tackled within a couple of previous Answers here (and is actually well covered elsewhere since it comes down to a fundamental of hand planing). Not to bury the lead the basic principle is actually incredibly simple: one just targets the high spots. That's literally it. This is the basis of so much in hand planing in fact; you'd use the same process if you were planing a crooked board's edges — target the high centre first on one edge, the high ends first on the opposite edge.
    – Graphus
    Feb 29 at 7:55
  • Makes sense, thanks! That’s what I suspected. If I may ask, how long of a straight edge should I use (as a proportion of the board)? Do I need a 2m one for a 2m board for best results?
    – stanch
    Feb 29 at 8:07
  • Ideally you do want a straight edge as long or longer than the surface being tested (doesn't have to be a formal straightedge, although definitely buy/make one or two of those [related link in next Comment]) Obviously this is not always practical, and neither is it necessary, but it is desirable when you can swing it... a 2m surface or edge are a good example of where, even in a professional workshop, a straightedge of that length is likely not available so one makes do with something shorter.
    – Graphus
    Mar 1 at 7:39
  • Don't forget the value of sighting down as at least a first check of straightness/flatness, coincidentally referred to in this Answer from just a few days ago, which also includes a little piece on shop-made straightedges as well as the all-important winding sticks.
    – Graphus
    Mar 1 at 7:42
  • I saw that! And it shows a 8-16' straightedge 😯
    – stanch
    Mar 1 at 8:15

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