The top of a rock ten handbreadths high and four handbreadths wide is its own private domain and one is liable for carrying onto it on Shabbos. Likewise a pit with a bank ten handbreadths high, even if the bottom of the pit is less than ten handbreadths beneath the level of the public domain. If we throw an object onto the rock, we’re liable even if it passes through exempt airspace to get there. A pit that becomes ten handbreadths deep when we remove the last shovelful of dirt: exempt. A rock thrown into a pit that is ten handbreadths deep, and thus makes it less than the minimum measure: one should be liable by the same logic as the previous case, but we’re not because the cases were left unresolved and so the halacha is lenient in both cases. Same for a board tossed onto four poles, thereby creating a private domain when it lands: exempt.