interior side 36 walls of the modified duct 40 although not necessary the opposed edges 44 45 can be cut as opposed lapped edges as best viewed in fig8 the modified duct 40 is collapsed laterally inwardly to bring the opposed cut edges 44 45 in engagement to form a semi arcuate duct 48 having seams 49 formed in the top 32 bottom 34 and interior side 36 walls of the arcuate duct 48 the outer side wall 38 remains intact although the interior portion of the outer wall 38 can be scored or slightly v grooved 47 in a vertical line in alignment with the apexes of the triangular cuts to facilitate bending and formation of a rounded outer side wall 38 thus the formed angular duct 48 contains a gradually rounded outer side wall 48 particularly suitable for efficiently redirecting air flow without interjecting substantial counteracting frictional forces to air flows through the fiberboard transition piece as viewed in fig7 the seams 49 are tightly secured by wrapping structural adhesive tape 46 around the wall sections including preferably the outer side wall 38 whereby structural integrity is produced and maintained in conjunction with the unitary outer side wall 38 the rounded transition piece can be interconnected to a linear duct unit by taping the juncture 25 therewith with tape 28 in the same manner as viewed in fig3 referring now to fig9 and 10 shown generally is an apparatus 60 particularly suitable for modifying linear fiberboard duct units 20.