From the book:

The standard GoF structure is shown below in Figure 1. The Client uses the Subject, which can either be a RealSubject or a Proxy. The Client does not know which, nor care. In the standard GoF structure, the Proxy points to a RealSubject. A variation of the proxy structure is to let the Proxy point to a Subject. This allows us to cascade proxies. We will examine cascading proxies in more detail at the end of the next chapter.

Figure 1. Structure of the proxy design pattern.