Franz Lehar,
Die lustige Witwe /
The Merry Widow (1905), Act 1 Ball-music. This single waltz strain is to be played offscreen under conversation, but also
ad libitum, meaning it can be left out (the same music appears later in the score -- a recording I listened to recently included the first statement but dropped the second one). The design is a 32-bar period (common in waltzes after about 1860, but also found regularly later on in marches, one-steps, and rags). The first 16-bar phase is one of those awkward units where period or sentence might apply equally well, depending on how you take the identical rhythms but different melodic shapes.
A triadic interval frame is established quite clearly -- circled at the beginning below -- and is returned to at the end of the first 16-bar phase -- circled at the beginning of the second system. In the second 16-bar unit, the triad is shifted upward -- circled across the second and third systems -- and overlaps with a simple scalar progress through the octave, with a registral drop on ^3. See the next example.
Here is a schematic of the triad frame (as proto-background) and a parsing of the octave line in the final six bars.
