After death the body goes through many chemical and physical changes.  One of these is termed the postmortem pH shift.   

In the early postmortem phase the body becomes acidic. This is because of continued metabolism and the lack of elimination of waste. As time progresses the body becomes more alkaline (or basic) due to proteins breaking down and the liberation of nitrogen from amino acids. 

So the postmortem pH shift is from Acidic to Basic or (and here’s the mnemonic) from A to B

I can’t stress strongly enough that its import to understand the chemical changes that are happening in a body to cause this but, Acid to Base or A to B is very helpful while you’re studying this topic.