In English poetry substitution, also known as inversion, is the use of an alien metric foot in a line of otherwise regular metrical pattern.^[1] 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:^[2]

To swell the gourd, and plump the hazel shells

Using '°' 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

In the first line the word that is stressed rather than is, which would be an unnatural accent. The first syllable of Whether is also stressed, making a trochaic beginning to the line.

John Milton used this technique extensively, prompting the critic F. R. Leavis to insultingly call this technique the Miltonic Thump.^[3]

Iambic substitution

Sometimes the opposite substitution, of an iamb in place of a trochee, is found, as in the following lines from Shelley's Ode to the West Wind:

Thou, from whose unseen presence the leaves dead

Are driven, like ghosts from an enchanter fleeing.

Here the words the leaves are an iamb (da DUM) in a place in the line where normally there would be a trochee (DUM da).