what is the rule of thirds?

While the rule of thirds is a good starting point for most, that's all it is; a starting point. It assumes that one is using a desirable room w/o even multiple dimensions. And that might be the best premise of this guide, using uneven dimensions that don't have even multiples of any dimension. For example a rule of fifths might work in a desirable room or a combination of odd multiples might work better in a less than desirable room.

Pryso, you beat me to it.
I'm not sure, but I believe that Peter Walker of Quad fame might have been one of,if not the first to promote the "rule of thirds".
If at all possible, if one can avoid even multiples from ceiling, floor as well as length and width it might be beneficial.

Yeah, 1/3 or 1/5 or 1/7....
Any distance that does not allow 'equal' different distances. (including the one you cannot change, the floor to speaker/speaker to ceiling.
Try to use odd fractions, and calculate the distances in like inches, for all the dimentions. When different dimensions equal the same distance.. bad. when the different dimensions equal all different lengths that are not multiples of 2 or 4 of each other GOOD.

fwiw the rule of thirds was the best setup for my Maggie 1.6s

The 'Golden Ratio" rule is just as important along these lines of thought. The ratio is 1.6 : 1 which was discovered during the Greek times thousands of years ago.

It can be used for room size propotions. For example if the room height is 8 feet, then the room width should be
1.6 times that or 13 feet. If the room width is 13 feet then the room length should be 1.6 times that or 21 feet. This will eliminate certain frequencies from being accentuated in the room.

If the distance from the front of the room to the speakers is 1 then the distance from the left side of the room to the right speaker is 1.6 times that. Same goes for the the other speaker. That will leave a relative distance of .6 between the side of each wall and the closest speaker.

Under these conditions, the distance between the two speakers is 1 as well as the distance from each speaker to the sweet spot is 1, thus forming an equilateral triangle.

This will tell you not only the relative room proportions, but where everything is positioned in the room.

This application of the golden ratio to audio rooms was first developed by George Cardas. You can check out more on this on the internet. Do a search for the golden ratio for audio uses