In English poetry substitution, also known as inversion, is the use of an alien metric foot in a line of otherwise regular metrical pattern. For instance in an iambic line of "da DUM", a trochaic substitution would introduce a foot of "DUM da".
Trochaic substitution
In a line of verse that normally employs iambic meter, trochaic substitution describes the replacement of an iamb by a trochee.
The following line from John Keats's To Autumn is straightforward iambic pentameter:
To swell the gourd, and plump the hazel shellsUsing '°' for a weak syllable, '/' for a strong syllable, and '|' for divisions between feet it can be represented as:
The opening of a sonnet by John Donne demonstrates trochaic substitution of the first foot ("Batter"):
Donne uses an inversion (DUM da instead of da DUM) in the first foot of the first line to stress the key verb, "batter", and then sets up a clear iambic pattern with the rest of the line
Shakespeare's Hamlet includes a well-known example:
To be, or not to be: that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune'
Here, that is emphasized rather than is, which would be a wrenched, or unnatural accent. The first syllable of Whether is also stressed, making it a trochaic beginning.