I use Cokin P Series filters for black and white photography, I think they are also called L (large). I do wish I hard started with the XL (100mm) filters now, but at the time my funds were limited. The reason is that I have a few camera lenses that need the larger size.
One thing that I need to know when using these filters is the exposure factor, the affect on the amount of light they have on reaching the film. For cameras with built in meters this isn’t a problem. My Mamiya doesn’t have a meter, so I use a hand held meter and adjust by increasing the exposure as required. To do that I need the table below!
These readings where taken by placing a filter in front of a Nikon D750, I also rounded to the nearest half a stop as my film cameras cannot even do halves. However I generally take a number of readings from neutral areas with a spot meter and factor in the filter affect to that calculation.