1929: one crash, a longer decline

Part of the share purchase is your money; the rest is borrowed. A price drop eats into your smaller stake while the lender's claim remains. That's how a modest movement can become a large loss relative to the cash you put in. Gains are magnified too.

Not every investor used a margin loan. The Dow Jones figures run from September 1929 to July 1932, almost three years. Black Tuesday is one date inside that decline.

Dow Jones Industrial Average, closing level

381.17 points

3 September 1929

Dow Jones Industrial Average, closing level

41.22 points

8 July 1932

Decline between those two closes

89.2%

over almost three years, between these two dated closes · our arithmetic

(381.17 − 41.22) ÷ 381.17 = 89.2%

These are index points, not dollars, even though the source's chart description uses a currency symbol in places. The two closes are the publisher's; the 89.2% between them is ours — the essay itself says “89 percent” — and it is marked as our arithmetic rather than as a figure anyone published. They cannot be converted into a total of debt or of investor cash losses. This is not a one-day Black Tuesday return. Borrowing to buy assets magnifies what a price change does to an investor's own stake, which is a separate mechanism from the index level; no share of investors who borrowed is claimed here. One stock market does not by itself explain the Great Depression.