The sestina is a 39 line poetry form first invented in circa 1200.
Its invention is often attributed to Occitan troubador Arnaut Daniel: https://en.wikipedia.org/wiki/Arnaut_Daniel
The sestina does not have a conventional rhyme scheme like the one you might find in a nursery rhyme or a ballad of abab, or aabb, etc.
Instead the sestina is comprised of blank or 'non-rhymes' which are a series of words that are repeated in a specific order. These words take the place of rhymes, sitting at the end of the line.
After six verses, the poem transforms these words into a concluding verse of three lines, called an 'envoi'. In the envoi, the words are repeated at the beginning of the line and at the end of the line. There is some flexibility as to how the envoi is structured, but the general rule is to have the even numbers on the right and odd numbers on the right.
This creates a 39 line structure:
Verse 1: 123456, Verse 2: 615243, Verse 3: 364125, Verse 4: 532614, Verse 5: 451362, Verse 6: 246531
Envoi:
2 5 4 3 6 1
This builder is a simple spreadsheet that allows the user to start the construction of the poem either from the envoi or from the start of the poem. It means that the architecture of what points they will then need to get to in the composition of the poem become immediately obvious, to help poets avoid potential hiccups in the construction of their poem before they start.
Insert the words 'non-rhymes' of your choice and watch them fill out across the page for you!