a. His Accidency
Tyler was born on Mar. 29, 1790.
He was the first Vice President to take over upon the death of a President. Having never been elected in his own right, his detractors questioned his legitimacy. Despite their objections, Tyler's interpretation of his Constitutional duties set a precedent for other Vice Presidents to follow.
Any lingering ambiguity about the transfer of presidential power was removed with the passage of the 25th Amendment in 1967.