How to build a octagon

With a compass guide a circle. Mark its center.

Mark at the ends of any diameter of the circle. This is the first two vertices of the octagon of the future.

Install solution of the compass equal to the diameter of the circle. Putting the needle of a compass in one of the marked in the previous step points, make the tick marks above and below the circle. Try to make them not too short, since they will need to overlap with cuts that you make in the next step.

Put the needle of the compass to another marked point and exactly the same make tick marks above and below the circle. If you draw a straight line between the points of intersection of the notches, then it will pass through the center of the circle, dividing the initial diameter exactly in half and is perpendicular to it.

Attach a line to two points and found the mark on the circle where it crosses built perpendicular. You have divided the circle into four equal parts, and you found points are the vertices of a square inscribed in a circle. The initial diameter and its perpendicular, was found in the previous step, are the diagonals of this square.

To complete the construction of a regular octagon, you need to find perpendiculars to the sides of the square.

Install solution of the compass equal to the side of the square. Place the needle of the compass at any vertex of the square and make notches on both sides at the outside of the circle.

Repeat the process with two vertices of the square, adjacent to the first. You should now have two points at the intersections of the tick marks.

Place a ruler so that it passes through any of the found points and the center of the circle. Make two marks on the circle where it intersects the received video. Repeat the same with second found point. You now have eight points, dividing the circumference into eight equal parts. This is the vertices of a regular octagon.

With a ruler connect consistently found all eight points. The build is complete.