25 February 1913: The income tax becomes constitutional

The US Secretary of State certified that the 16th Amendment had been ratified, giving Congress the power to levy an income tax without dividing it among the states by population. The first modern income tax that year had a top rate of just 7% and applied to very few people. Rates and reach grew dramatically over the following decades. Few amendments have had a bigger effect on investors' returns.

The lesson: Taxes are one of the biggest drags on long-term returns, and the rules can change.