PL ), ▁so ▁in ▁September ▁ 2 0 0 0 , ▁T roll tech ▁made ▁the ▁Unix ▁version ▁of ▁the ▁Qt ▁libraries ▁available ▁under ▁the ▁G PL , ▁in ▁addition ▁to ▁the ▁Q PL . ▁T roll tech ▁continued ▁to ▁require ▁lic enses ▁for ▁developing ▁propriet ary ▁software ▁with ▁Qt . ▁The ▁core ▁libraries ▁of ▁K DE ▁are ▁collect ively ▁lic ensed ▁under ▁the ▁GNU ▁L G PL , ▁but ▁the ▁only ▁way ▁for ▁propriet ary ▁software ▁to ▁make ▁use ▁of ▁them ▁was ▁to ▁be ▁developed ▁under ▁the ▁terms ▁of ▁the ▁Qt ▁propriet ary ▁license . ▁ ▁Second ▁ ▁series ▁ ▁Begin ning ▁ 2 3 ▁October ▁ 2 0 0 0 , ▁the ▁second ▁series ▁of ▁releases , ▁K ▁Des ktop ▁Environment ▁ 2 , ▁introduced ▁significant ▁techn ological ▁improvements . ▁These ▁included ▁D CO P ▁( Desktop ▁CO mm unic ation ▁Pro tocol ), ▁K IO ▁( an ▁application ▁I / O ▁library ), ▁K P arts ▁( a ▁component ▁object ▁model , ▁which ▁allows ▁an ▁application ▁to ▁embed ▁another ▁within ▁itself ), ▁and ▁K HTML ▁( an ▁HTML ▁rendering ▁and ▁drawing ▁engine ). ▁ ▁Third ▁series ▁ ▁The ▁third ▁series ▁was ▁much ▁larger ▁than ▁previous ▁series , ▁consisting ▁of ▁six ▁major ▁releases ▁starting ▁on ▁ 3 ▁April ▁ 2 0 0 2 . ▁The ▁API ▁changes ▁between ▁K ▁Des ktop ▁Environment   2 ▁and ▁K ▁Des ktop ▁Environment   3 ▁were ▁compar atively ▁minor , ▁meaning ▁that ▁the ▁K DE ▁ 3 ▁can ▁be ▁seen ▁as ▁largely ▁a ▁continu ation