system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 What is delta 8

What is delta8? What is the is the difference between delta8 and delta9? Is it safe? and what makes delta8 legal and delta9 cannabis illegal?
gpt
The 2018 Farm Bill defined marijuana as "cannabis that contains more than .3% of THC-9" and hemp as cannabis that contained less than that.  At the time that the Farm Bill was passed it was believed that THC was one compound (THC-9).  As it turns out it is scientifically possible to characterize two different forms of THC.  Those are THC-8 and THC-9.  THC-8 and THC-9 are essentially identical to one another, both chemically and in function.  For all practical purposes they are the same chemical.  

At the time that the Farm Bill was written nobody knew about THC-8 so they just used the formal, scientific name for THC, which at the time was THC-9.  However, because the Farm Bill defined marijuana on the basis of THC-9 content, some people have been making an argument that cannabis that contains high levels of THC-8 and <.3% of THC-9 wouldn't be considered marijuana and so would be considered hemp at the Federal level.

This is a fairly academic argument because:

1) There isn't a strain of cannabis that would be legal under this definition and it would take some time to produce one that would be.

2) Even if there was a strain of cannabis that met this definition, both CBD and THC-8 are still illegal to sell in consumer products.

3) The Farm Bill didn't affect anything in the Federal Analogue Act and so THC-8 is likely still illegal as a THC-9 analogue anyway.

That doesn't mean people aren't selling it as a "legal alternative" to marijuana.  But people do that with a large number of other substances that are illegal under the Analogue Act as well.  

The Federal government doesn't *generally* prosecute marijuana related crimes in states where it is legal, and marijuana prosecutions in states where it is not legal are also quite rare.  That's why people are "allowed" to sell it - its not legal there just isn't much enforcement on it.

As to how THC-8 is being produced from hemp - its possible to convert CBD to THC-8, which is also clearly illegal to do at the Federal level.