Triangle graph paper, A4
Equilateral triangles with 10 mm sides on A4, a tessellation grid for geometric design, quilt piecing and pattern work.
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Triangles and hexagons are the same grid
An equilateral triangle grid and a hexagon grid are duals of each other: join the centers of the hexagons and you get triangles, join the triangle centers and you get hexagons. Which one you print depends on what you are drawing on it rather than on the underlying geometry. Triangles suit piecing and tessellation, where the triangle itself is the unit; hexagons suit maps and cell-based layouts, where the hexagon is.
Where the 10 mm is measured
The spacing is the side length of each triangle, which is also the distance between adjacent grid points along any of the three directions. The perpendicular height of each triangle is smaller, about 8.66 mm for a 10 mm side. This trips people up when they measure a printed sheet vertically and conclude the scale is wrong. Measure along a triangle edge, not across a row.
Quilting and seam allowance
For quilt piecing, the drawn triangle is the finished size, not the cut size. You add seam allowance outside the line, conventionally a quarter inch in the US and often 7.5 mm elsewhere. Charting on true-size paper only helps if the printer has not rescaled the page, which is worth verifying once with the calibration sheet before you cut fabric to a printed pattern.
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.