I came across "bis-tris" buffer in Rodney Boyer's Biochemistry Laboratory book and tried looking it up. Then I found that there's bis-tris methane and bis-tris propane that are both buffers. What exactly are they used for and are the usages different?
They have different acid dissociation equilibria and so buffer at different pH.
pKa1 pKa2 Bis-Tris methane 6.5 - Bis-Tris propane 6.8 9
The two titratable secondary amines of Bis-Tris propane make it usable over a wider pH range.