Torino Manuscript - Ricercari Libro 1(17.c)
56. Ricercar terzo[con tre Soghetti]
= Il Primo Libro di Capricci, Canzon francese, e Recercari fatti sopra Diversi Soggetti
et Arie in Partitura.(1626)
Now that you mention it, the four-step notation used by Frescobaldi is for organ. It is often used for pieces that strictly keep the voices. It is often used for music with strictly vocal parts. Although this notation was already obsolete in Bach's time, he adopted this old method in “Die Kunst der Fuge”.
bar1-30 : A part, Theme 1
bar30-44 : B part, Theme 1 + 2
bar44-86 : Theme 1 + 2 + 3
http://aeolianconsort.chakin.com/Ricercari_Libro1_di_Torino.htm#40
