High Low Harmony
tderz: The opening a/o middle game principle Harmony of high & low moves refers usually to moves on the 3rd and 4th lines.
The principle wants to teach that the combination of moves on the 3rd and 4th lines gives you also the combination of stability and territory.
Moves on the 3rd line carry the stability component, while moves on the 4th line add to the prospects for territory.
Moves only on the 4th line get eventually more netto territory by adding the safety of 3rd line moves and vice versa.
tderzGood: _ The high - low combination brings safety and more territory (advice for 10-6k)
[10] Wrong: All black moves are on the third line and thus low - this is not efficient nor much safer.
Imagine or create similar examples with only moves on the 4-th line.
[10] Source of diagrams (adapted, TD): Guo Juan, Teaching Go at different Levels, Nov. 2002
Guo Juan call this efficiency principle the "High - Low combination". (advice for 10-6k)