The grain is the top outermost layer of the hide. It’s the part you see. This layer of the hide is also the most powerful, its fibers are very dense and tightly “woven”. And that makes sense if you think about a cow in the wild. It needs the outer layer to be tough to contend with predators, fences, etc.
The junction is where the fibers start to loosen up a bit and the grain transitions to the split or suede part of the hide.
The split as it is usually included essentially suede. Its softer, looser fibers make up the bulk of the hide. It is also the weakest part of the hide.
But it’s what happens from here that matters most.
Once the top layer of the hide has been altered in certain ways, it is no longer full-grain leather. This is also where the semantics start to get a little dicey. Once the top layer has been altered, you could end up with top grain, corrected grain, or genuine leather.
The term top grain leather normally refers to the top layer of the cowhide that has been minimally adjusted, possibly sanded down a bit to hide imperfections. Some people may tell you that top grain leather is the same as full-grain, but in my experience, that is simply not true.
Here’s one way you can tell. Anyone selling top grain leather is happy to have you believe that it’s the same as full-grain leather. But anyone selling full-grain leather makes sure to let you know it’s full-grain leather and they don’t market it as the top grain. It seems that industry practice is that top-grain leather means at least something has been done to the hide, usually some sanding of the surface.
So why does this matter? Two reasons. First, sanding the hide makes it weaker. We’re building bags to last a very long time, we don’t want to weaken the hide. Second, the leather loses some character. With full-grain leather, you can see the leather surface in its natural state – imperfections and all. For example, if the cow ran into a barbed-wire fence, you may see a little scar on the hide. Full-grain leather also allows the natural marbling, texture, and variety of color in the hide to come through fully.
Corrected grain leather is usually more altered than top grain leather. It could just be sanded, but it is often buffed or embossed, or resurfaced in some way. Why would anyone do this? Money. You can take a cheap hide with lots of flaws and resurface it to make it appear uniform.