Orthocenter
The orthocenter of a triangle is the point where all three of its altitudes intersect.
See Orthocenter of a triangle.
Animation showing that in any triangle, the centroid, circumcenter and orthocenter are always collinear  they always lie on a straight line, called the Euler line.
How to construct the orthocenter of a triangle with compass and straightedge or ruler. The orthocenter is the point where all three altitudes of the triangle intersect. An altitude is a line which passes through a vertex of the triangle and is perpendicular to the opposite side. A Euclidean construction
Definition and properties of orthocenter of a triangle
An overview of the various centers of a triangle
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