The diagonal cells of a matrix from upper left to lower right.

A web diagonal member joining the top and bottom chords of a truss, and taking its greatest stress when not less than one half of the span is covered by the live load.

A tension member of a truss, sloping upward towards the nearer end of the span. Main diagonals in iron bridges are not adjustable.

the elements in a square matrix ranging from the upper left to the lower right corners of the matrix. Communality estimates are inserted in the main diagonal of a correlation matrix when performing a factor analysis. When performing a principal component analysis the main diagonal elements of the correlation matrix will be 1s.

the set of squares which bisect the board from corner to corner (a1 to h8 or a8 to h1) middle game the middle phase of the game, between the opening and endgame

In linear algebra, the main diagonal of a square matrix is the diagonal which runs from the top left corner to the bottom right corner.