Why does the global variable of a class start with M by default?

One day idle bored, suddenly remembered why everyone is accustomed to the global variable using m start, so seeked, looked up some information, although not I want, but also summed up.

On the StackOverflow, someone asked:

Why does most variables on Android tutorial start with ' m '?

The following answer is in the Android source code site has a description of the code, including:

Follow Field naming conventions

Non-public, no-static field names start with M.

Static field names start with S.

Other fields start with a lower case letter.

public static final fields (constants) is all_caps_with_underscores.

Website: source.android.com/source/code-style.html

Why does the global variable of a class start with M by default?