According to the automaker, the tradition started in the '50s but has roots that started with the marque's beginnings in 1948. Colin Chapman, the founder of the British brand, named the first Lotus road vehicle he designed and built the Mark I, choosing to use Roman numerals to represent the number.
That tradition went on with the subsequent models up until Mark X. While the next car in line would have been called the Lotus Mark XI, the motoring media back in 1956 seemed to have preferred calling it the Lotus XI instead. Chapman chose to drop Mark from the naming system moving forward.
However, to avoid the confusion of the Arabic numeral '11' that's visually similar to Mark II, Chapman decided to spell out the name, therefore giving birth to Lotus Eleven. This also marked the birth of the tradition that we all know today for all Lotus road cars..