Being pedantic, OLE1 was the predecessor. OLE2 used COM for its plumbing. Wikipedia has an article on the file format [1]. It was quite nice. It works like

Being pedantic, OLE1 was the predecessor. OLE2 used COM for its plumbing. Wikipedia has an article on the file format [1]. It was quite nice. It works like an uncompressed zip file with transactional updates. Earlier Word document formats were much worse. They were a dump of Word's memory contents. Saving and loading was very quick though! [1]: https://en.wikipedia.org/wiki/Compound_File_Binary_Format https://en.wikipedia.org/wiki/Compound_File_Binary_Format