KG stands for Knight of the Garter

Edward III of England started the Order of the Garter in 1348. Those that are inducted into the Order are known as a Knight of that order. This is not just an ordinary old knighthood. Members are assigned positions in the order of precedence, coming before all others of knightly rank, and above baronets. Knights of the Order of the Garter get the rather fancy initials after their name, "KG". The last knight inducted was "Prince William, Duke of Cambridge KG KT". Yes, that is his full official name.  When an individual is entitled to use multiple post-nominal letters like "KG", those of the Order of the Garter appear before all others except "Bt" (Baronet), "VC" (Victoria Cross) and "GC" (George Cross). You may notice that William also is a KT which means "Order of the Thistle" as well. A fun fact about William is that he also became the 1000th Knight in the Order of the Garter since 1348. His father, Prince of Wales, was the last knight inducted into the Order of the Garter exactly 40 years before him. The Order of Garter is the oldest order of chivalry in the world.  The Order has limited membership to 24 people. However like Prince William he got in through a loophole that doesn't count his membership against the total number of members, the loophole allows British Royal Family a special class that is not limited in size called a Supernumerary Knight. There are 8 Supernumerary Knight currently. There is also another unlimited membership size of knights in the class of Stranger Knights. Those are foreign Knights, usually Kings or Emperors. The King of Sweden is a Stranger Knight as is the Emperor of Japan. Historians continue to debate the origins of the Order of the Garter. The popular story is that Edward III was dancing with Joan, Countess of Salisbury when her garter fell to the floor.