* The y coordinate (height) does not matter, but make sure your x and z are precise.
To link portals from the overworld to the nether, they must be in correct locaitons The Nether coordinate system is 1/8th the one of the Overworld's. Place the Overworld portal, hit F3 to find out the coordinates. Travel to the Nether and build the nether portal on the coordinates obtained from the calculator.
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Portal image credit http://gearcraft.us/finding-a-portal-in-real-life-story/