thomasteepe

It's not from another source, and in the specific combination shown in the illustration it's "my own method". But there is, of course, material on traditional mind maps like the Buzan book, literature on problem solving like Polya's "How to Solve It" and many many others, and general material on thinking and writing. For me, the concept of "concatenating units of thought" via the boxes was a surprise, with an unexpected impact it had on my thinking processes, and I do not know of a method that uses this as a key point.

For me, it's a natural consequence of filling a box line-wise vertically from top to bottom. If you want to use the sheet layout variant with 3 columns and boxes of variable height rather than a fixed 3x3 layout, the vertical scheme seems again more natural.

But whoever feels more comfortable with a horizontal scheme should do so.