Point counting is a technique for estimating three-dimensional structural data from two-dimensional data, in this case a flat image under the microscope. In point counting, each cross hair represents a specific area. For example, on the slide to the right, the area in the purple shaded box is represented by one adjacent cross hair on the grid, marked with an orange dot.

So, you can estimate how much space in the lung is taken up by septal tissue by counting the number of cross hairs that touch septal tissue. This technique works because of geometrical relationships. Mathematically, one can show that the ratios of volume (three-dimensional structures) are correlated with ratios area (two-dimensional structures).