Donald Trump stood up straight when he got off the plane in Saudi Arabia and was greeted by King Salman. But Barack Obama had earlier bowed to Saudi King Abdullah, “contorting nearly to a 90-degree angle.”  Aha! So there you go.
But when President Trump was presented with the Collar of Abdulaziz Al Saud by King Salman, he had to bow his head while the king put it on. So you could say that, in a way, Trump did bow to the Saudis, though not at a 90-degree angle.
All eyes are watching to see if President Trump will bow to the Jesuit Pope Francis on May 24th at the Vatican. The Protocol of Millard Fillmore demands that Trump not bow to the Pope.
When Millard Fillmore was no longer President, he took a tour of Europe. Late in 1855, he arrived in Italy. As a former U.S. President, arrangements were made for Fillmore to have a private audience with the Pope. Shortly before being presented to “His Holiness,” Fillmore learned he would be expected “to kneel and kiss the hand of the Pope, if not his foot.” 
Our Millard, normally a calm man, in this case lost his temper! “Fillmore stormed in consternation: ‘…if this was the case, I must decline the honor…'” of an audience with the Pope. 
Millard Fillmore, even as an ex-President, still felt he represented the honor and dignity of the United States. To bow before any form of royalty would be un-American, contrary to this nation’s founding principles. Thus, Millard Fillmore would never have bowed before the Japanese emperor, nor any other emperor, king, prince, or Pope. 
Fillmore’s firm stance caused the Pope to back down. The Pope received Fillmore and offered neither hand nor foot for salutation. 
——- Sources ——-
 “GREAT AGAIN: Unlike Obama, Trump doesn’t bow to Saudi king”, by Kyle Olson. The American Mirror (online), May 20, 2017. http://www.theamericanmirror.com/great-unlike-obama-trump-doesnt-bow-saudi-king/
 Millard Fillmore, by Robert J. Rayback. Newtown, CT: American Political Biography Press, 1959