Rule 17 of 31 · Chapter IV — Keep Tools Sharp
Keep the reference face flat
Why this rule exists
A chisel's back and a plane iron's flat face are the reference the edge is measured from. If that face isn't dead flat, especially the first inch behind the edge, you can't get a truly sharp, straight edge no matter how you grind the bevel. Flatten it once, properly, and every future sharpening rides on that trued surface. Skip it and you'll chase a wire edge forever and wonder why the tool never quite bites clean.
In practice
On a new chisel or plane iron, flatten the back on coarse then progressively finer stones until the area right behind the edge is uniformly polished, mirror-bright. You only need the working inch or two, not the whole length of the tool. Once it's flattened, never grind the back again; you sharpen only the bevel side from then on. Keep the back away from coarse abrasives entirely so it stays dead true for the working life of the tool.
When it doesn't apply
The ruler trick puts a tiny back bevel on plane irons to save flattening a large or pitted face, and that's a fine shortcut for bench planes. Chisels used for paring and chopping need a genuinely flat back, so don't back-bevel those. Match the method to the tool's job.