I believe at the time, St. Petersburg was founded in order to replace Moscow, which had been destroyed at the time.
Well, the best answer is: we don't really know for sure; the event wasn't documented properly.
There are some guesses though, and one of them is the following: the Liberty Bell cracked in 1824, when Marquis de Lafayette was visiting. Another time mentioned often is 1835.
Answer:
In modern Western societies, social stratification is typically defined in terms of three social classes: the upper class, the middle class, and the lower class; in turn, each class can be subdivided into the upper-stratum, the middle-stratum, and the lower stratum.