Heads of state and government are typically called Mr. President or Madam Prime Minister. So when President Donald Trump used a post Thursday on his social media site to speak directly to Russia's president,

President Trump, who just hours earlier had suggested that Ukraine’s president was the central impediment to peace, issued a rare public rebuke of Vladimir Putin: “Vladimir, STOP!” Trump wrote on his social media platform. “I am not happy with the Russian strikes on KYIV. Not necessary, and very bad timing.”