Please answer a question about the following article about Treaty:

Since the late 19th century, most treaties have followed a fairly consistent format. A treaty typically begins with a preamble describing the contracting parties and their joint objectives in executing the treaty, as well as summarizing any underlying events (such as a war). Modern preambles are sometimes structured as a single very long sentence formatted into multiple paragraphs for readability, in which each of the paragraphs begins with a verb (desiring, recognizing, having, and so on).

What is the beginning of a typical treaty called?
a preamble