In many historic calculations of net worth, individual fortunes weren't measured in isolation; instead, they were compared to the GDP of the US at the time. When the US economy was much smaller than it is today, the wealthiest families and individuals carried more weight in the economy than others during more prosperous times. One of the richest Americans of all time is widely considered to be Standard Oil founder John D. Rockefeller, whose wealth, at its peak in 1937, was equal to 1.5% of the country's GDP.
He was wrong. A trillion is a million millions-twelve zeroes-but in his defence, it is an unfathomable number. If Musk has $1 trillion (US), his wealth would hover somewhere between the gross domestic product of the Netherlands-$1.2 trillion (US)-and Switzerland-$936 billion (US)-making him the equivalent of the world's twentieth largest economy. The International Space Station, the most expensive structure ever built, was co-funded by fourteen countries, including the United States, Canada, Russia, and Japan, and it cost around $150 billion (US).