Q16. "I have been on the net for about an hour reading all I can find about informal changes which have occurred in the Constitution during the last 200 years. Which kind of change, formal or informal, have been most important in our history?"
A. An "Informal Change" or "Informal Amendment" to the Constitution is one where the interpretation of the Constitution is changed by a Supreme Court ruling, rather than by a formal constitutional amendment.
There are some truly momentous Supreme Court decisions in our history - Marbury v Madison, Dredd Scott, Miranda, Brown v Board of Education, Roe v Wade. Recently, I find FCC v Pacifica (1978) fascinating, Hustler v Falwell (1988) is also interesting, and Webster v Reproductive Health (1989) is interesting in what it could have been but wasn't.
For example, in the Plessy v Ferguson, the Court ruled that blacks and whites could have separate public facilities, including schools, as long as those facilities were equal. This "separate but equal" interpretation of the 14th Amendment was overthrown by the Brown v Board ruling that declared the separate but equal theory unconstitutional.