This form was invented by Thomas Horton of Allpoetry.
It's called a RIDOTTO, from the Italian for "reduced." In a ridotto, you choose a number of syllables for your first line (x). Your second line should be a perfect rhyming couplet with one more syllable (x+1). The third line takes on a new rhyme, and has one fewer syllable than the first one (x-1). Line four rhymes with line 3, and has one fewer syllable (x, or [x-1]+1). This continues until the poem is reduced to a couplet of one syllable followed by two.
Here's an example:
FIRST KISS (a ridotto)
The way the soft light broke ----> (6)
Through the branches of the oak ----> (7)
Gave the day a glow ----> (5)
That you and I would know ----> (6)
Brought to an end ----> (4)
Our time to pretend ----> (5)
And we shared ----> (3)
Though we were scared ----> (4)
Desire ----> (2)
Like a fire ----> (3)
Rife ----> (1)
With life ----> (2)
© Thomas Horton, All Rights Reserved.
You may start with any number of syllables you like; as such, the poem may be of any length, as long was the second line of each rhyming couplet has one more syllable than the first line, and the first line of each subsequent couplet has one fewer syllable than the first line of the previous couplet.
Near rhymes don't count; all end-rhymes should be full/strong/masculine.
I have re-phrased the instructions thus:
1. Pen a line with any number of syllables.
2. For the next line, add one syllable and rhyme with the preceding line.
3. Subtract two syllables and choose a new rhyme word.
Repeat instructions 2 thru 3 until instruction 3 would create a zero syllable line.
Rhyme pattern aabbccddee..etc
No metric requirement.
Visual template (for beginning with an even number of syllables)