What are Master pages in InDesign?
Master pages are pages which are designed and then applied to which ever pages you want. There can even be multiple types of master pages. They become helpful when you need to add page numbers or chapter names to pages, when you want to add background images to a document, or style your document in many different ways.
Where are Master pages in InDesign?
In the layout menu, scroll to the top of the "Pages" tab. Here you will find the default master pages. I like to have a variety of master page types in a template ready to use.
By right clicking on this default, you will see a lot of options.
Select "New Master" and a dialog box will pop up. One of the options is "Based on Master", which drops down into a menu of the other Masters pages. By selecting "A-Master", you will see the contents of "A-Master" appear on your new master.
Editing "A-Master", add a type box to the page and assign it paragraph style "Page Numbers". Go to the "Type" menu, and scroll down to "Insert Special Character" > "Markers" > "Current Page Number". You will see an "A" appear in the text box.
Now if you click on "B-Master", you will see the page number there as well, but it will not be editable from this master page. You can add other symbols or text to "B-Master", and it will not be reflected in "A-Master". You can also make some masters pages with different background colors and image backgrounds.