The rondeau cinquain originally had a refrain of five verses and a total length of 21 which became the norm in the 15th century. The rhyme scheme is AABBA–aab–AAB–aabba–AABBA.
The modern version has the refrains shortened and the restatements are reduced to just the first two or three words of the first line, which stand as short, pithy, non-rhyming lines in the middle and at the end of the poem. These half-lines are called rentrement. If derived from the erstwhile 21-line rondeau cinquain, the result is a 15-line form with the rentrements in lines 9 and 15 (rhyme scheme aabba–aabR–aabbaR). This 15-line form became the norm in the literary rondeau of the later Renaissance, and is known as the “rondeau” proper today
I discovered first this form on 2 January 2016 thanks to a talented fellow blogger and poet Dajena aka moonskittles. You can discover her poetry here