Use of varying line lengths with varying numbers of syllables per line. Critic Gay Wilson Allen identified the Whitman "envelope": a short beginning line, long middle lines, and a short ending line.

Where the mockingbird sounds his delicious gurgles, and cackles and screams and  weeps,
  Where the hay-rick stands in the barnyard, and the dry-stalks are scattered, and the   brood cow waits in the hovel,
 Where the bull advances to do his masculine work, and the stud to the mare, and the   cock is treading the hen,
    Where the heifers browse, and the geese nip their food with short jerks;                                                                 Where the sundown shadows lengthen over the limitless and lonesome prairie,
 Where the herds of buffalo make a crawling spread of the square miles far and    near;
 Where the hummingbird shimmers . . . . where the neck of the longlived swan is   curving and winding
Where the laughing-gull scoots by the slappy shore and laughs her near-human