An answer to a question someone asked on a different site:
Leverage was an incredibly important aspect of Livermore's obscene success in trading. You ask how much leverage did he use. Before The Great Depression in October 1929, Livermore reportedly had $20 million liquid. He used his relatively illiquid assets (yacht, property) to collateralize against another $10 million. He put up $30 million as margin to short nearly $500 million in stock. Let's think about that for a minute. If the stocks he shorted went up just 3%, he'd lose half his net worth ($500,000,000 x 3% = $15,000,000). This is something he did THROUGHOUT his trading career - not just 1929.
The reason he often lost so much after making so much is because he experienced these adverse price movements after betting a multiple of 10 - 20 times everything he had. If I had to guess, his leverage averaged out between 10x - 20x. I cannot fathom using this amount of leverage trading equities. Timing would have to be nearly perfect to avoid an extreme drawdown. The more leverage you use, the better timed your trades/investments need to be to avoid getting run over. It is incredibly challenging. A hundred years ago prices probably didn't flash higher or lower in milliseconds as they do today, which may have made the extreme leverage factor easier to implement, but still a major challenge.
In a nutshell, Livermore nearly perfected the art of leverage in his time. Nearly.
Leverage used before the crash that caused The Great Depression found in Pg 314 - 315 in The Man Who Sold America Short in 1929 by Tom Rubython