Possible pcf algebras
  There exists a family $\{B_{\alpha}\}_{\alpha<\omega_1}$ of sets of countable ordinals such that   o $\max B_{\alpha}=\alpha$,   o if $\alpha\in B_{\beta}$ then $B_{\alpha}\subseteq B_{\beta}$,   o if $\lambda\leq \alpha$ and $\lambda$ is a limit ordinal then $B_{\alpha}\cap\lambda$ is not in the ideal generated by the $B_{\beta}$, $\beta<\alpha$, and by the bounded subsets of $\lambda$,   o there is a partition $\{A_n\}_{n=0}^{\infty}$ of $\omega_1$ such that for every $\alpha$ and every $n,$ $B_{\alpha}\cap A_n$ is finite. 
Thomas Jech and Saharon Shelah