I have already mentioned Markowsky’s Theorem (1976): every chain-complete poset is a dcpo. This is a non-trivial theorem, and I’ve given you a proof of it based on Iwamura’s Lemma and ordinals in a previous post. Maurice Pouzet recently pointed me to P. M. Cohn’s book *Universal algebra* (1965), where you can find the same theorem already! Cohn’s proof is very different and does not rely on Iwamura’s Lemma. Let me describe it in the full post.