Triangle generates meshes with subparametric quadratic elements if the -o2 switch is specified. Quadratic elements have six nodes per element, rather than three. `Subparametric' means that the edges of the triangles are always straight, so that subparametric quadratic elements are geometrically identical to linear elements, even though they can be used with quadratic interpolating functions. The three extra nodes of an element fall at the midpoints of the three edges.
In .ele files produced by Triangle with the -o2 switch, each triangular element has six nodes. The first three nodes are the corners in counterclockwise order, and the fourth, fifth, and sixth nodes lie on the midpoints of the edges opposite the first, second, and third corners, respectively.