Tank and Must: two legends in one. On the one hand, Tank, the essential and dandy watch that first appeared in 1917. On the other, Must, a concept that rewrote the conventions of luxury in the 1970s. Together they form a signature, the Tank Must whose design and movements are in constant progression. Its style has evolved, coming closer to that of the historic model, while a solar-powered calibre puts into practice Cartier’s commitment to the environment and biodiversity. This SolarBeat™ movement was two years in development and has a lifespan of sixteen years between services. The light reaches the photovoltaic cells under the dial through invisible perforations in the Roman numerals. In a similarly sustainable vein, the strap is made from an innovative material that repurposes apple waste.