The first Jumbo bricks, as made by Samsonite, were not compatible with the regular system bricks. The next version, as found in Europe, was better. It was exactly 3x the size of the regular bricks. It was still not fully compatible though, because it could not attach to system bricks above or below. This was solved with the Duplo design. These bricks are 2x regular size and fully compatible. The way they did that was so good that the idea was patentable.
The patent describes a brick that has to work with the regular system brick. It states: “When making a building structure from such elements it may in certain cases be of interest to use larger elements than the normal, i.e. elements which are both of greater height and of greater width than the normal elements. Also to arouse the interest of smaller children it may be an advantage that they can start playing with a building set with larger elements which are easier to handle for small children, who may then later on become interested in more advanced and detailed structures composed of smaller elements.” (quoted from USA patent)
After that, a formal description of the new bricks is given. The main characteristics are:
- The brick is twice the size of the basic module
- The brick has tubular primary studs. The internal diameter of the studs corresponds to the external diameter of the inner tubes of the smaller base element. In this manner the inner tubes of the small elements will engage the tubular primary studs of the large elements and thus ensure an efficient coupling.
- The inner tubes of the larger brick are made slightly shorter than the height of the sidewalls, so that these tubes will not be in the way of the primary studs of a small element coupled to the underside of the large element. An alternative option is to give the inner tubes recesses that receive the primary studs of the smaller brick (the castellated tubes).
This solves all the problems that were encountered when combining basic bricks with Jumbo bricks.
The Danish patent application was filed on 29 Nov. 1967.
It was published as
You can find them here: worldwide.espacenet.com
The larger Quatro bricks (4x regular size) use a similar system with tubular studs. However the Quatro bricks did not use a shorted inner tube. These bricks have multiple “Duplo sized” inner tubes. I suspect this solution was chosen to optimise the clutch strength of the bricks.
A slightly modified mold version of the Duplo brick was required to fit onto the Quatro system. The supports between the tubes had to be raised a little so the Quatro studs fit beneath them.