When the feet are accented on the last syllable,—that is, when the verse is iambic or anapestic,—an extra syllable may be added at the end of a line.
U |U U |U _ | U
I stood on the bridge at midnight,
U U | U |U U _ |
As the clocks were striking the hour;
U U | U | U _|U
And the Moon rose o'er the city,
U | U | U _ |
Behind the dark church tower.
—Longfellow.
U | U |U | U | U | U |
Girt round with rugged moun[tains], the fair Lake Constance lies,
U | U | U | U | U |U |
In her blue heart reflect[ed] shine back the starry skies;
U | U | U | U |U | U |
And watching each white cloud[let] float silently and slow,
U | U | U | U | U | U |
You think a piece of heav[en] lies on our earth below.