What was the name of the 1920s U.S. stock-market practice of buying shares with borrowed money?

The 1920s U.S. stock-market practice of buying shares with borrowed money was called buying on margin.

Buying on margin allows an investor to purchase securities using a combination of personal funds and a loan from a broker. During the 1920s, widespread margin buying helped increase demand for stocks and allowed investors to control positions much larger than their available cash would otherwise permit.

The strategy magnified gains when prices rose, but it also magnified losses when prices fell. Brokers could issue margin calls requiring investors to provide more cash or sell securities. Forced selling during the 1929 collapse added to downward pressure as investors tried to repay loans and satisfy those calls.

Buying on margin is different from short selling. A margin buyer generally borrows money to buy an asset, while a short seller borrows the asset itself and sells it, hoping to buy it back later at a lower price. Both involve borrowing, but their market positions are opposite.