Lego pieces of all varieties constitute a universal system. Despite variations in the design and the purposes of individual pieces over the years, each remains compatible in some way with existing pieces. Lego bricks from 1958 still interlock with those made presently, and Lego sets for young children are compatible with those made for teenagers. Six bricks of 2 × 4 studs[40] can be combined in 915,103,765 ways.[41]

Each piece must be manufactured to an exacting degree of precision. When two pieces are engaged, they must fit firmly, yet be easily disassembled. The machines that manufacture Lego bricks have tolerances as small as 10 micrometres.[42]Dimensions of some standard Lego bricks and plates[43][44]

Primary concept and development work for the toy takes place at the Billund headquarters, where the company employs approximately 120 designers. The company also has smaller design offices in the UK, Spain, Germany, and Japan which are tasked with developing products aimed specifically at their respective national markets. The average development period for a new product is around twelve months, split into three stages. The first is to identify market trends and developments, including contact by the designers directly with the market; some are stationed in toy shops close to holidays, while others interview children. The second stage is the design and development of the product based on the results of the first stage. As of September 2008 the design teams use 3D modelling software to generate CAD drawings from initial design sketches. The designs are then prototyped using an in-house stereolithography machine. These prototypes are presented to the entire project team for comment and testing by parents and children during the "validation" process. Designs may then be altered in accordance with the results from the focus groups. Virtual models of completed Lego products are built concurrently with the writing of the user instructions. Completed CAD models are also used in the wider organisation for marketing and packaging.[45]

Lego Digital Designer was an official piece of Lego software for Mac OS X and Windows which allowed users to create their own digital Lego designs.[46] The program once allowed customers to order custom designs[47] with a service to ship physical models from Digital Designer to consumers; the service ended in 2012.[48] Lego's website now recommends BrickLink Studio.[49]