Alignment of the Body Text across the spread29 Nov 2019
The baseline in typography is the imaginary line on which most letters sit. The descenders of letters will extend below this line. Also, the body of some letters such as the J and Q in some typefaces will also extend slightly below this baseline.
The purpose of using the baseline grid is to make sure that the main text will align properly across columns or across the spine of the pages when seen in a double page spread. There can be something very uncomfortable looking when the text appears on a different alignment. This is always particularly obvious when the text reaches the bottom of the page. When the book is printed on thin paper, the text may be seen through the page; if the lines of text coincide (back to back), then this will be less noticeable.
There are several steps and various options for aligning the text to a common baseline grid.
See Attention to Detail in Typography with InDesign