What happens when two polarizers are placed in a straight line, one behind the other?

When two polarizers are placed in a straight line, they allow light to pass only if their directions of polarizations are exactly 90° apart.   

To add, a polarizer or polariser is an optical filter that lets light waves of a specific polarization pass and blocks light waves of other polarizations.

Arise 8 run 1 this means go 8 up and 1 across, which is steepest

It's because 3 if it was 10 it wouldn't look right do it goes up to 4d