One of the mistakes you often see in books, is that the book isn't printed "line on line". Depending on the opacity of the paper it is printed on, you can glimpse the other side of the paper through the page, and if the lines on the opposite site isn't aligned with the ones you are currently reading, it can be really frustrating and make it more difficult to read.
This is one of the reasons we use baseline grids.
By adding a baseline grid with a specific leading to your document, you can make sure that all your text is locked to that grid, and by that, assure the final result is printed "line on line".
The document
I have created a document with a specific top, bottom, inner and outer margin. My main text paragraphs are made with Meta Serif with a 10pt font size on a 13pt leading (in short terms, 10/13pt).
Since the main text paragraphs have a 13pt leading, this is the increment I will use for the baseline grid. The baseline grid is set up in Preferences » Grids.
Preferences » Grids is the place you set up your document baseline grid.
After setting up the baseline grid, I want to make sure that the margins and the text aligns. As you can see in the next image, there is a gap below the last line of the baseline grid, and the bottom margin. In this case the distance is 0.925mm, which i will I will add to the current Start value of the baseline grid, to move the whole grid a bit down on the page and align it to the bottom margin.
Adjusting the baseline grid's start level to align with the bottom margin.
Now in some cases it would be necessary to adjust the documents top margin, to make space for an extra line, but in this case it isn't necessary. If it was the case, the margin should of course be adjusted on the master pages, to affect all pages.
The styles
In this case I am using Meta Serif, 10/13pt, so the first thing I will define in the main paragraph style is the font, the font size and the leading.
The next thing I will define, is that the paragraph style should lock all lines to the baseline grid. This is done in the Indents and Spacing section. In reality, if my leading was set to 10pt, and all lines was set to lock to the baseline grid, the result would still be a 13pt leading. If the style's leading was set to 14pt though, the end result would be a leading of 26pt.
Locking all lines to the baseline grid.
Once you have the baseline grid set up, you can easily use it for many other things. E.g. small subheadings you want to control the space above and below. In the next image you will see a subheading taking up three lines of space, and adjusting the space by 2/3 above and 1/3 below. This is done by locking the lines to the baseline grid and lifting it from the baseline by 8.667pt using Baseline Shift in the Advanced Character Formats section.
I will refer to my screencast on "subheadings the right way" for more information on why I choose to do my subheadings this way.
To lift locked text a specific amount of space from the baseline, you can use Baseline Shift.
Grid based design
Amongst other things you can use the baseline grid for are placement of images, page numbers, running headers etc. Consistently using the same spacing makes the page design more logic - which is why you probably also want to look more into grid based designing.
I just want to add a final comment to using grids for designing. Grids are a great tool and can really help organizing things in a design. Aligning everything perfectly is a great start, but in the end it is the human eye that matters.
If you place images in the baseline grid, you really want the top of the image to optically align with the top of the text, which means you really want the top of the image to begin somewhere else than in the grid, probably in the x-height of the text.
Fine tuning a grid based design is all about optical alignment - the human eye is what matters in the end!
It also means in bigger headings, uncials etc, you probably want to let serifs in A's, V's etc break the left text frame edge a little, to optically align the left edge of the text.
InDesign gives you tools to achieve some of this, but I will save that for upcoming posts in the better typography series.
Check out The Grid System for a lot of templates, articles, tools and inspiration on grid based design.