The most popular and modern probes that are based on the fractionator method involve marking only one type of event. To estimate number with the optical fractionator, the cell-top or some other unique point is marked. To estimate length with spaceballs, the intersection of the string with the surface of the sphere is marked. To estimate surface with the isotropic fakir, the intersection of the membrane or the objects with probing line segments is marked; and to estimate volume with the Cavalieri/point-counting probe, points are marked.

It can be useful, however, to combine the marking of more than one type of these events to obtain estimates.

### The Selector

The selector uses a probe for particle-volume, along with a probe for percentage of tissue by volume made up of the particles, to come up with an estimate of the number of particles.

### Estimating the Chord Length of Alveoli

Many useful probes combine the marking of an intersection to estimate surface or length with marking points to estimate volume. Examples are ‘using the image plane as the probe‘ for estimating length and Cycloids for Sv or Merz-semi-circles for estimating surface. Let’s take a look at an example of marking intersections and points to characterize alveoli in respiratory research, please click here.

### Estimating Width of Slabs

Estimates of volume using points, and surface using intersections, can be combined to estimate the width of regions that are in the form of slabs, such as the cortical ribbon or blood vessel walls. For details please click here. This is an alternative method to directly estimating the thickness with Orthogonal Intercepts.