sort - Alphabetical lists - horizontal or vertical?

When listing a large set of data alphabetically Is it better to list them horizontally or vertically? Craiglist lists states in columns vertically. Another site I've been using http://www.rent-acoder.com/ lists the project tags in rows horizontally. Vertically makes sense because you scroll down naturally to view more text. But horizontally also makes sense because humans read left to right, then top to bottom. What is more natural from a ux point of view?

Edit:: Also, would it matter how much content there is to display? If there were a list of say 4 columns x 2 rows, it seems cumbersome to scan up and down, up and down alphabetically. And if it's 2 columns by 4 rows, it seems cumbersome to go left right, left right.

Answer

1. 
   

   Vertical lists are much easier to scan than horizontal lists, because all items are aligned to the same line, so when you're looking for an item, you don't need to read the entire word - you can quickly scan the first letters and get directly to the item you need. See how much less time it takes you to locate items beginning with P on Craigslist (within a state) than on the Rent a coder site.

   
   



2. 
   

   Even when the sorting is horizontal, users will often scan it vertically, because of the Gestalt law of Proximity, unless the horizontal spacing within the rows is much smaller than the vertical spacing between the rows. The Rent a coder site is actually a very good example of this - you can see how the columns are much more salient than the rows. And the checkmarks make it even worse. So users begin to scan vertically, then they see that it doesn't make sense, and then they may or may not try to figure this out and see that the sorting is actually horizontal. So you'll want your sorting to be consistent with your design, rather than contradict it.