I have used XXX (and ZZZ) in my own code because they are the only tokens that satisfy these 3 things. 1. It stands out clearly visually in the code. 2. It is

I have used XXX (and ZZZ) in my own code because they are the only tokens that satisfy these 3 things. 1. It stands out clearly visually in the code. 2. It is not normally a keyword, variable or string in my code. 3. I can type it quickly and comfortably using only my left hand and two fingers, while my right hand controls the cursor to navigate (or is moving to the keyboard after cursor navigation). I usually start typing it during the interval as my right hand begins moving left over the keyboard to prepare to type the comment that will follow. I suspect XXX and ZZZZ are not simply conventions that are picked up by exposure to other people's code, I think they represent a local maximum of convenience for these three criteria. Try typing any other token (e.g. TODO) and see if it satisfies these criteria as well as XXX or ZZZ.