<P> The tower 's tilt began during construction in the 12th century , caused by an inadequate foundation on ground too soft on one side to properly support the structure 's weight . The tilt increased in the decades before the structure was completed in the 14th century . It gradually increased until the structure was stabilized ( and the tilt partially corrected ) by efforts in the late 20th and early 21st centuries . </P>
when did the leaning tower of pisa start to lean
during construction in the 12th century