Diplomacy: Theory and Practice, 6th ed.  –  Online updating pages

Chapter 5: Packaging Agreements

p. 77, side letters: These have a long history. I have just discovered that one accompanied the Treaty of Blois of 19 April 1572, which created an edgy defensive alliance between England and France. Nervous of having a commitment to come to the aid of the Protestant English in the event of an attack by Catholic Spain written into the treaty, the French were willing only to provide an assurance on this point in a private letter written by King Charles IX. This was accepted by the English negotiators in return for a tacit retreat by the French in their support for Mary Queen of Scots.