Triangulate Modifier
The Triangulate modifier converts all faces in a mesh (quads and n-gons) to triangular faces. It fulfills the exact same function as the Triangulate tool in Edit Mode.
Options
Quad Method
Beauty
Split the quads in nice triangles, slower method.
Fixed
Split the quads on their 1st and 3rd vertices.
Fixed Alternate
Split the quads on their 2nd and 4th vertices.
Shortest Diagonal
Split the quads along their shortest diagonal.
Longest Diagonal
Split the quads along their longest diagonal. This is the preferred mode for cloth simulations.
N-gon Method
Beauty
Arrange the new triangles nicely, slower method.
Clip
Splits n-gons using an ear-clipping algorithm (the same method of tessellation used for viewport display).
Minimum Vertices
Minimum number of vertices a face must have to be triangulated. For example, setting this value to 5, will prevent triangulation of Quads and only triangulate N-gons.
Keep Normals
When using custom normals, try to preserve the same shading as before triangulation.