It is evident that one foot can be substituted for another if the accent is not changed.  Since both the iambus and the anapest are accented on the last syllable, they may be interchanged.  The trochee and the dactyl are both accented on the first syllable and may, therefore, be interchanged.

There are some exceptions to the general rule that in substituting one foot for another the accented syllable must be kept in the same part of the foot.  Occasionally a poem in which the prevailing foot is iambic has a trochee for the first foot of a line in order that it may begin with an accented syllable.  At the beginning of a line the change of accent is scarcely noticeable.

_ U | U   _  | U  _  |U  _ |
Over the rail my hand I trail.

_  U  | U   _ |   U   _    | U   _ |
Silent the crumbling bridge we cross!

But if the reader has once fallen into the swing of iambic verse, the substitution of a trochee will bring the accent at an unexpected place, interrupt the smooth flow of the rhythm, and produce a harsh and jarring effect.  Such a change of accent is justified only when the sense of the verse leads the reader to expect the changed accent, or when the emphasis thus given to the sense of the poem more than compensates for the break in the rhythm produced by the change of accent.

Another form of metrical variation is that in which there are too few or too many syllables in a foot.  This generally occurs at the end of a line, but may occur at the beginning.  If a syllable is added or omitted skillfully, the rhythm will be unbroken.

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.

—­Adelaide A. Procter.

In the second illustration the extra syllables have the same relative position in the metrical scheme as in the first, though they appear to be in the middle of the line.  The pauses fill in the time and preserve the rhythm unbroken.

When the feet are accented on the first syllable—­as in trochaic or dactylic verse—­a syllable may be omitted from the end of a line as in the second and fourth below.