plurality of flat increments 14a and top section 12 may comprise or include a plurality of flat increments 12a so long as the overall transition duct is a generally flared so as to gradually diverge this insures that the height of outlet 22 is always greater than the height of inlet 20 air inlet 20 is formed by the four edges of top section 12 side section 16 bottom section 14 and side section 18 at one longitudinal side of duct 10 air outlet 22 usually is formed by the four edges of top section 12 side section 16 bottom section 14 and side section 18 at the other longitudinal side of duct 10 the overall height of outlet 22 always is greater than the height of inlet 20 accordingly the height of end sections 16 and 18 at outlet 22 is greater than their height at inlet 20 end sections 16 and 18 are preferably curved outwardly sideways toward outlet 22 such that the overall width of outlet 22 is also usually greater than the width of inlet 20 bottom section 14 is of a greater longitudinal length than top section 12 imaginary planes contacting the edges of inlet 20 and outlet 22 when extended above duct 10 form an angle a of between 1 u00b0 and 60 u00b0.