The Music Stem / The Musician's Library / Composer's Handbook / Metric Modulation
Composer's Handbook
Metric modulation changes tempo by making a note value in the old tempo equal a different one in the new.
Let the quarter-note triplet of the old tempo become the new quarter note.
At quarter = 60, quarter-note triplets move at 90 per minute. They become the new beat: quarter = 90.