What makes a piece of music difficult? This album introduces a pair of riveting, technically ambitious, historically important, and never-before-recorded polyphonic masses of the fifteenth century, performed by an ensemble that has developed a new approach designed to honor the music's variety and unflagging intensity. Both works stand out for their notational complexity. They are also exceptional for the skill they demand of the performers: we find hair-raising rhythms, intricate counterpoint, and long melodic phrases. Rather than mitigate these challenges with a large ensemble and a generous acoustic, this recording enhances them: the performances are one-on-a-part, featuring energetic tempi, close miking, and minimal reverberation. This approach is unforgiving-but together with bright vowels and a flexible vocal technique, it has the benefit of allowing the music to come across with uncommon directness and clarity.