In the previous chapter we learned the basics of how to make LaTeX documents to create and present research findings. So far we have only learned how to create short documents, like articles and slideshows. For longer and more complex documents, like theses and books, a single LaTeX markup file can become very unwieldy very quickly, especially when it includes knitr code chunks as well. Ideally we would segment the markup file into individual chapter files, for example, and then bring them all together when we compile the whole document. This would allow us to benefit from a modular file structure while producing one presentation document with continuous section and page numbering. To do this we can take advantage of LaTeX and knitr to separate markup files into manageable pieces. Like directories, these pieces are called child files, which are combined using a parent document.