Consider the problem of finding a velocity vector field 
and a pressure
scalar field p solution of
The variational formulation is
with
Let us call 
and 
, then, with obvious notations
Two well known discretizations are used:
The P1-P1 stabilized element where by all variables u and p are continuous and piecewise linear on a triangulation but the triangulation for u is obtained by dividing each triangle of the triangulation of p into 3 triangles by joining the vertices to the center of gravity of each triangle.
The second method does the same but divides the triangles of p into 4 subtriangles for u by joining the mid-edges.
Figure 2: The P1-bubble /P1 element (left) and the P1-iso-P2/P1 element (right)