When you list the average transmission of a polarizer, what is the difference between single, parallel, and crossed?

The value of transmission for a single polarizer refers to the percentage of the incident light that passes through one single polarizer. The value of transmission for parallel polarizers refers to the percentage of the incident light that passes through two polarizers, where the axis of polarization for each polarizer is aligned in the same direction. The value of transmission for crossed polarizers refers to the percentage of the incident light that passes through two polarizers, where the axis of polarization for each polarizer is separated by a 90 degree angle. The average value stated refers to the actual average of all transmission values from 400 to 700nm.