I'm not a frame builder . Please correct me if I'm wrong here . But , as I recall from the 1970's & 1980's . . .
( and of course , as always , you can't see any of this from the outside of the tube )
Double Butted - means that each end of the tube has a thicker wall than the middle of the tube .
Triple Butted - means that while each end has a thicker wall than the middle of the tube , one end has an even thicker wall than the other end . This term was undoubtedly invented by an advertising executive .
Quadruple Butted - means that the ends have thick walls , the middle has a thin wall , and there are transition areas in between . Plus , in addition , the thicknesses at one end are different from the thicknesses at the other end . Again , I'm guessing that the advertising department went crazy with this one . I recall seeing pretty illustrations in the magazines , of just where each tube was what thickness .
Theoretically , a quad-butted tube could have wall thicknesses of ( just making this up as an example ) ; 2mm , 1.5mm , 1mm for the long middle part , 1.25mm , 1.75mm .
Raoul Delmare
Marysville Kansas