Meter.
In literature, mood refers to the general atmosphere the writer creates and the feeling, thoughts, and emotions this atmosphere evokes in the reader. In the case of poems, this mood is linked to the metric which is the rhythm in a poem, which is also called the pattern of beats and depends on the number of syllables in each verse and the number of stressed/ unstressed syllables in each verse. In this way, the metric sets the pace of the poem and a slow pace would create a melancholic or sad atmosphere while by including more unstressed syllables, while the use of more stressed or accent syllables would create a more dynamic and cheerful mood. Thus, one of the techniques used by poets to create mood is the meter.