1.3.5. How CA Certificates Establish Trust

CAs validate identities and issue certificates. They can be either independent third parties or organizations running their own certificate-issuing server software, such as the Certificate System.
Any client or server software that supports certificates maintains a collection of trusted CA certificates. These CA certificates determine which issuers of certificates the software can trust, or validate. In the simplest case, the software can validate only certificates issued by one of the CAs for which it has a certificate. It is also possible for a trusted CA certificate to be part of a chain of CA certificates, each issued by the CA above it in a certificate hierarchy.
The sections that follow explains how certificate hierarchies and certificate chains determine what certificates software can trust. CA Hierarchies

In large organizations, responsibility for issuing certificates can be delegated to several different CAs. For example, the number of certificates required may be too large for a single CA to maintain; different organizational units may have different policy requirements; or a CA may need to be physically located in the same geographic area as the people to whom it is issuing certificates.
These certificate-issuing responsibilities can be divided among subordinate CAs. The X.509 standard includes a model for setting up a hierarchy of CAs, shown in Figure 1.6, “Example of a Hierarchy of Certificate Authorities”.
Example of a Hierarchy of Certificate Authorities

Figure 1.6. Example of a Hierarchy of Certificate Authorities

The root CA is at the top of the hierarchy. The root CA's certificate is a self-signed certificate; that is, the certificate is digitally signed by the same entity that the certificate identifies. The CAs that are directly subordinate to the root CA have CA certificates signed by the root CA. CAs under the subordinate CAs in the hierarchy have their CA certificates signed by the higher-level subordinate CAs.
Organizations have a great deal of flexibility in how CA hierarchies are set up; Figure 1.6, “Example of a Hierarchy of Certificate Authorities” shows just one example.