Programmer's Guide > Programming and Scripting Techniques > Line Numbering in Arbortext Editor and Arbortext Publishing Engine > Line numbering application building reference > The Layout file and document type
  
The Layout file and document type
The Layout document type defines the .layout file, which is produced by the Arbortext formatting engine and written to the .aptcache folder when line numbering is applied to a document. The .layout file specifies the structure of the document as it will appear in composed output, and defines where the atipl tags will appear.
The format of the .layout file is defined by the following document type definition. A typical declaration would be structured in this way:
<?xml version=1.0?>
<!DOCTYPE layout PUBLIC "-//Arbortext//DTD Layout 1.0//EN"
"layout/layout.dtd">