The tandem WW domain of the human formin-binding protein 21 (h-FBP21 tWW) consists of two WW domains separated by a flexible linker. It can bind target sequences in two different orientations and the flexibility of the linker additionally allows the two WW domains to adopt various relative orientations to each other. As consequence, the elucidation of possible complex structures for the h-FBP21 tWW is very challenging. Here, we present two complex structures for the h-FBP21 tWW and a proline-rich sequence from its natural binding partner, the core-splicing protein SmB/B’. Showing parallel (‘6’) and antiparallel (’14’) binding orientation, the two structures also differ in the relative positioning of the WW domains. For further instructions regarding the files, please refer to ‘README’.