Difference between rubber skirts

[GoneFishingLTN]

Can someone help me understand what is flat rubber, living rubber and what are the common grades I see fine, medium and large. 
 

is old school rubber usually living rubber? I see a lot listed as old school rubber 

[Jig Man]

As far as I know living rubber is all it has ever been called.
 

Medium flat used to be the only way it came.  If you stored it very long the strands would stick together and make the whole thing unusable.  Round is a lot better in my opinion.  I have some which has been stored for years and it is as good as when I got it.  
 

Fine is ok but I don’t have much use for it.

 

I have never seen large.

[Bluebasser86]

I have some large cut rubber I use to get lots of flare from my jig skirts, kind of a mop jig look. They really like it at times. I like the flat cut rubber for spinnerbaits. 

[Bdnoble84]

Flat vs round and width of the cut

[Munkin]

To me they are all a PITA to separate the strands so I don't use it.

 

Allen 

[smalljaw67]

Living rubber is just a term applied to round rubber. Old school rubber is a term used for flat or square rubber that was used in the 70s and 80s. Most of what is commonly used is medium or fine, large rubber is often called "regular".  Round rubber was liked by a lot of anglers because it has a lot of movement. It fans out faster than silicone so when you're actively working a jig there is a lot more movement with the skirt. Flat rubber doesn't have the fast flare like round rubber but it moves a lot more water. In addition to displacing a lot of water it also creates a bulky profile. Square rubber now comes in tabs and is similar in shape to silicone. It is a little thicker so you get a larger profile while using the same amount of material. It has roughly the same action as silicone but is bulkier and displaces more water. Flat rubber, especially the fine grade, gained popularity when Edwin Evers won a Bass Masters Classic. He caught some good fish on an Andy's finesse jig with a skirt made with fine flat rubber.