Triangle paper, one-inch
Equilateral triangles with a one-inch side. At this size the printed sheet is a real template rather than a plan.
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A one-to-one patchwork template
Sixty-degree patchwork — equilateral triangle quilts, tumbling blocks, thousand pyramids — can be drawn, cut and checked directly against a sheet printed at true size. Anything you can lay a rotary ruler across, you can verify before touching fabric. At smaller triangle sizes you are working on a plan and scaling up; at an inch you are working on the thing itself.
Large triangles for teaching geometry
One-inch equilateral triangles are also the right size for classroom work on tessellation, symmetry and angle sums, where children need to write inside each shape and colour it in. At 10 mm the triangles are too small for a pencil-written label, and the exercise turns into a test of fine motor control rather than of geometry.
Finished size, not cut size
As with all patchwork drafting, the grid shows finished dimensions — the quarter-inch seam allowance is added when cutting, never drawn on the plan. A one-inch finished triangle is cut at roughly 1¾ inches on a side once allowance and blunted points are accounted for. Check that against the 60° markings on your ruler rather than measuring the drawn triangle and adding by eye.
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.