Alliteration occurs frequently throughout "The Tyger," usually as a way of representing the poem's events and settings through sound. For example, in line 1 (and in its repeat in line 21), the repeated /b/ sound of "burning bright" makes the phrase more colorful and vivid. That is, the close repetition of the sound creates a kind of intensity that is meant to conjure the intense bright light emitted by a fire. The effect is both aural and visual.
In line 4 (and again in line 24), an /f/ sound repeats in "frame" and "fearful." Here, the narrator is expressing a cautious sense of awe at the abilities and powers of whoever created the tiger. The use of "frame" relates to artistry and craftsmanship, and the deliberate placement of the alliteration creates a sense of meticulousness, skill, and attention to detail. In other words, the creation of the tiger, though difficult to understand, is undoubtedly deliberate.
Line 5 uses two /d/ sounds in "distant deeps" to create a sense of depth that perhaps relates to the idea of hell as an underground place. The following stanzas describe the tiger being created in an atmosphere of high pressure and heat, and the use of alliteration here helps to foreshadow that atmosphere. It is as though the poem itself is under a similar pressure, causing a chemical reaction in the words that makes them take on matching forms—and sound alike.
Later, line 11's alliteration brings the idea of a heart beating to sonic life, with the two /b/ sounds in "began" and "beat" creating a pulsating, rhythmic sound.
The alliteration of line 16 links "daringness" and "deadliness" together conceptually (and also recalls the /d/ sounds in line 5). This instance of alliteration underscores a key part of the poem's argument: humans should not reject the tiger outright as a fearsome creature, but rather appreciate the bravery of the creator for bringing it into being.