Perfboard layout paper, A3
Holes at 0.1 inch pitch across A3, for boards too large to plan on a single A4 sheet.
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Large boards need the whole layout visible
At 0.1 inch pitch an A4 sheet holds about 80 by 110 holes. That is a respectable board, but a full Eurocard, a backplane, or anything with several ICs and a lot of interconnect runs past it. Splitting the layout across pages is where routing mistakes happen, because the connections that cross the join are exactly the ones you stop tracing. A3 holds around 115 by 165 holes.
Major lines every ten
On a large grid of identical holes, counting is the problem. A heavier line every tenth hole turns counting into reading, the same way the bold line every ten stitches works on a cross stitch chart. Ten also matches the way component pitches are usually described, since a 0.1 inch grid means ten holes is exactly one inch and you can measure the layout with a ruler as a check.
Verify the scale with a real component
The sheet is only useful if it matches the board, and print scaling silently breaks that. Lay an actual DIP chip or a length of pin header on the printed page and check the pins land on holes across their full length, not just at one end. Over an A3 sheet a small scaling error accumulates into a visible drift. The stripboard sheet is the version with track markings.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.