The top of the rubber is to be no higher than ten inches (25.4 cm) above home plate. From 1903 through 1968, this height limit was set at 15 inches, but was often slightly higher, sometimes as high as 20 inches (50.8 cm), especially for teams that emphasized pitching.
The lowering of the mound in 1969 was intended to "increase the batting" once again, as pitching had become increasingly dominant, reaching its peak the prior year; 1968 is known among Baseball historians as "The Year of the Pitcher". This restrictive rule apparently did its job, contributing to the hitting surge of modern baseball.
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The pitchers mound was created in the late 1800's. It was put in to create a downward angle for the pitches, and to help the pitcher generate speed when sloping downhill. The height was originally 10 inches, but was raised to 15 in 1903. Over the years, hitting went down, and the mound was lowered back to ten inches.
Following the 1968 season the pitching mound was lowered from 15 inches to 10. Pitchers had reached unprecedented success in 1968, highlighted by Bob Gibson's 1.12 ERA. Lowering the mound helped the hitters.
Prior to 1903, a pitching mound in MLB could be any height desired by the home team -- even zero if they didn't want any mound at all. From that year until 1968, a REGULATION mound had to be 15 inches higher than home plate. In that year, that height has been reduced to 10 inches. It has been a long-standing cheat for a home team with good pitchers to add height to the mound. Doing so has almost never been caught, simply because mound height (too high or too low) benefits (or hinders) both teams equally -- thus, nobody bothers to challenge it.