Draw the knot tight. Note the symmetry of the knot: the two rope pieces on the left emerge together side by side from under the loop, while the two rope pieces on the right emerge together above the other loop. This appearance confirms you have tied the square knot correctly.
Note that a square knot will pull very tight under a load and can become difficult to untie. For this reason, the similar reef knot is often used when the knot is to be untied soon.
If the knot is tired incorrectly, with two overhand knots that do not oppose each other, a granny knot results. A granny knot will eventually slip under tension. See the next photo for a granny knot.
