According to Lyon, the bref double has a single form with a fixed rhyme scheme and, most distinctively, only the first two quatrains share a final rhyme (unrhymed lines shown as "X"): AXBC XAXC AXAB AB.
According to Turco, the bref double does have three rhymes, but the scheme is such that the first two of must appear twice in the first three quatrains—all of which end with the third rhyme, with five unrhymed lines. In Turco's given example the scheme looks like this: AXBC XAXC BXXC AB.
However, other variants are possible, such as: AXXB CXXB ACXB AC.
Both sources agree that there is no requirement of meter in a bref double, though all lines must be consistent in length.