A key can (and usually does) change during a composition. Of course, here we are talking about what we defined as the key of a passage (chapter Key of passage) and not what we defined as the key of a composition as a whole (chapter Key of composition). In other words, we are saying that usually we can divide a composition into few passages, each written in a different key. The most straightforward way to find these keys (and their passages) is by looking at the note names being used in the composition and comparing them with the diatonic tones of different keys (in the same way as it was described in chapter Key of passage).
Later in the book, we shall learn that what might look like a brief passage written in a different key than its neighboring passage, could turn out to be written in the same key as the neighboring passage. This is called tonicization and it happens when secondary chords are used. The secondary chords and tonicization will be explained later in the book. The important thing to remember now is that only when we perform the analysis of harmony for a composition, then we can determine definitely the keys in the composition. This is the reason why we shall postpone the rest of the story about keys until later in the book when we shall know how to perform harmony analysis. Here, we shall explain difficulties that we may encounter while finding the keys in a composition by looking at the note names being used:
If only few note names appear in a passage, it might be difficult to tell exactly in which key the passage is written. Few note names can belong to different keys. For instance, the group of note names C, D, G and A can belong to four different major keys: C major key, G major key, F major key and Bb major key. Sometimes, composers deliberately explore such possibilities to incorporate a degree of tonal ambiguity in a passage.
The change of a key might be followed by the change of a key signature but does not have to. Sometimes, a key signature does not change together with the change of a key. When lots of accidentals are present in a composition, it might be difficult to tell what is happening. Is the composition still in the same key even it has many altered tones (chromaticism)? Or did the composition switched to some other key even the key signature was not changed?
Furthermore, determining keys in a composition only by looking at the note names being used might sometimes be problematic. Any major key and its relative minor key share the same seven diatonic note names. Thus, it might be impossible to distinguish between the relative keys only by looking at the note names from the composition. Later in the book, we shall learn that other important clues exist that help in recognition of a key. These clues are chord progressions, especially cadences. Such clues are discovered during harmony analysis.