A standard model of ZF need not be transitive, of course, and Joel David Hamkins' answer to [Large cardinal axioms and Grothendieck universes](https://mathoverflow.net/questions/12804/large-cardinal-axioms-and-grothendieck-universes?rq=1) gives Tarski sets as an interesting example.


I should clarify since the terminology is not entirely standard. By a standard model I mean what Wikipedia does, and what Joel David Hamkins does in his answer to [Standard model of ZFC](https://mathoverflow.net/questions/124720/standard-model-of-zfc): the set membership relation $E$ of the model is the actual set membership relation $\in$, restricted to sets in $M$. 


Does existence of a standard model imply existence of a transitive one? Existence of a Tarski set does imply existence of a Grothendieck universe. But am I right to suspect that if there is a standard model, then the ordinals in the minimal model are not a true initial segment of the ordinals, so the minimal model is not transitive, and we can cut down to where that minimal model is the only standard one? If I understand Guest289 correctly, his argument shows my guess is wrong, since a minimal model is transitive. Do I understand that correctly? Or does this come back to unclarity abut what is a standard model?


Anyway, does existence of a transitive model have higher consistency strength than existence of a standard model? I would not be surprised if the axiom of choice lays a role here but I do not know if it does.



What is the consistency strength of a standard model of ZF versus a transitive model?
What precisely do you mean by a standard model? An $\omega$-model? (That is, a model whose set of natural numbers is isomorphic to $\omega$.) Or a $\beta$-model? (That is, a model whose ordinals are well-ordered.) 


If the latter, the [Mostowski collapse theorem](http://en.wikipedia.org/wiki/Mostowski_collapse_lemma) tells us any such model is isomorphic in a unique way to a unique transitive model. If the former, the existence of an $\omega$-model has consistency strength strictly weaker than the existence of a transitive one. (This follows, for instance, from [absoluteness](https://math.stackexchange.com/a/33708/462) considerations: Any transitive model of $\mathsf{ZF}$ has as elements some $\omega$-models.)


It is true that there are $\beta$-models whose ordinals do not form an initial segment of the true ordinals, even if the membership relation of the model is true membership. For instance, if $V\_\alpha$ is a model of set theory, consider any of its countable elementary substructures.


Note that models whose membership relation is true membership are $\beta$-models, so they are isomorphic, via the collapse, to transitive models. The point of the Mostowski collapse is that there is a natural rank that we can assign to the elements of a standard model $M$, so that if $M\models x\in y$ then the rank of $x$ is strictly smaller than the rank of $y$. (If the membership relation of $M$ is true membership, we can use as rank the true set-theoretic rank of the sets in question.) A straightforward transfinite recursion then allows us to replace the elements of $M$ by copies that form a transitive model, simply by defining $\pi:M\to V$ by $\pi(y)=\{\pi(x)∣M\models x\in y\}$. Choice plays no role here.


