What are the advantages of a multi-column grid?  
Multi column grids provide flexible formats for publications that have a complex hierarchy or that integrate text and illustrations. The more columns you create, the more flexible your grid becomes. 

How many characters is optimal for line length? 
The optimal line length is considered to be 50-60 characters long, including spaces. 

Why is the baseline grid used in design?
 Baseline grids serve to anchor all (or nearly all) layout elements to a common rhythm 

What are reasons to set type justified? ragged (unjustified)? 
Justifying text allows the page to appear more organized and structured.  

What is a typographic river?
When justifying text, large spaces in between text start to form and they connect from line to line creating a "river" look throughout the paragraph. 

What does clothesline, hang-line or flow line mean? 
Hang-line is when body text can "hang" from a common line.