Triangle graph paper
A grid of equilateral triangles, horizontal lines crossed by two sets of 60° diagonals.
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Tessellation and pattern design
The triangular grid is the densest of the three regular tilings, and it contains the hexagonal and rhombic grids inside it: join alternate vertices and you get hexagons, pair adjacent triangles and you get rhombi. That makes it the most flexible starting point for pattern work, because you can derive the other layouts without changing paper.
Quilting and patchwork
Half-square and equilateral triangle patchwork blocks are much easier to plan on a true triangular grid than on squared paper, where you have to imagine the diagonals. Set the spacing to your finished triangle side length and the printed sheet becomes a 1:1 template. Note that equilateral triangles and half-square triangles are different problems. Half-square triangles come from cutting a square corner to corner and belong on ordinary graph paper, since their legs sit on the square grid. This sheet is for true 60 degree triangles, the ones used in hexagon and diamond piecing, where the geometry does not fit a square grid at all. As always the drawn line is the finished size and the seam allowance goes outside it.
Relation to isometric paper
Triangle paper and isometric paper are the same lattice rotated by 30°. Isometric has vertical lines and 30° diagonals; this has horizontal lines and 60° diagonals. Choose isometric for drawing three-dimensional objects, and triangle for flat pattern work. The practical test is whether your drawing has a vertical. A box, a building or a piece of furniture has verticals that should stay vertical, which is what isometric paper gives you. A tessellation, a quilt layout or a triangular truss diagram has no privileged vertical direction, so the rotated lattice sits more naturally under it and you waste less of the sheet.
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.