Plus Plus are wonderful building blocks from Denmark. http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=plus+plus They pack small and are perfect for playing while waiting at restaurants.

My 7 year old wanted an additional shape block to complement his Plus Plus set. He explained that the block needed a hole in the center. He designed the Plus4 part and I adjusted the tolerances.

The Plus4 fits well with the original Plus Plus. Unfortunately when made with PLA, the tolerances and durometer do not allow the Plus4 to mate with another Plus4 piece. Perhaps if made of a softer durometer polymer, eg. NinjaFlex, or if the dimensions are adjusted, an entire building set of Plus4 could be created.

UPDATE: Plus4 can be printed with PolyFlex filament! Without any adjustments to the dimensions, Plus4 pieces can now fit with with other Plus4 pieces. See the photos of the orange sample pieces.

Based on the Plus Plus blocks from Denmark.