Firstly, lets describe a regular coffee filter basket.
Below is a photo of both what a regular unpressurised and pressurised filter basket may look like from the inside.
With a regular unpressurised basket, you are able to see through each one of those tiny pin holes in the basket.
In a pressurised basket, you are unable to see through any of the holes. Instead, if you turn your basket over, you will actually find only 1 hole. In most cases it will usually also be stamped with the words “dual floor” or “dual wall”, meaning it has a fake bottom.
Pressurised coffee baskets come standard with most domestic branded coffee machines such as Breville, Krups and Sunbeam. Forcing coffee through a single pin hole “pressurised basket” will create a faux or fake crema from already stale pre-ground coffee. This “crema” is essentially aerated and whipped coffee. Creating this fake crema comes at the expense of taste regardless of whether you are using pre-ground or freshly ground coffee! This is why The Espresso School recommends the use of regular unpressurised baskets.
Sunbeam, Krups and Breville use a standard three lug 51mm diameter portafilter/group handle which can be interchanged between machines. Krups manufactures a non-pressurised basket - model MS0907163, which can be used in most 51 mm portafilter machines.