ɪn ˌoʊˈɛs slˈæʃ θɹˈiːhˈʌndɹɪd sˈɪksti, ˌaɪbˌiːˈɛm ɛkstˈɛndᵻd ðɪ ˌoʊvɚlˈeɪ fəsˈɪlᵻɾi ʌv ˈɪbldɚ baɪ ɐlˈaʊɪŋ ɐn ˌoʊvɚlˈeɪ pɹˈoʊɡɹæm tə hæv ˌɪndᵻpˈɛndənt ˌoʊvɚlˈeɪ ɹˈiːdʒənz, ˈiːtʃ wɪð ɪts ˈoʊn ˌoʊvɚlˈeɪ tɹˈiː.
In OS/360, IBM extended the overlay facility of IBLDR by allowing an overlay program to have independent overlay regions, each with its own overlay tree.