Which direction is better for sorting multi-column lists?

Please note, I'm not asking how to implement or code a multi-column list.

There are two ways to sort multi-column lists: horizontal and vertical. Many of the Microsoft System.Web.UI.WebControls have a RepeatDirection property that offers these two options. I'm sure other frameworks also offer these direction options.

Below are samples of the two options:

Horizontal sorting (alphabetical)

* Ash      * Beech      * Cedar
* Date     * Elm        * Fir
* Grape    * Hawthorn   * Ivy

Horizontal sorting (numeric)

1. Ash      2. Beech       3. Cedar
4. Date     5. Elm  开发者_运维知识库       6. Fir
7. Grape    8. Hawthorn    9. Ivy

Vertical sorting (alphabetical)

* Ash      * Date    * Grape
* Beech    * Elm     * Hawthorn
* Cedar    * Fir     * Ivy

Vertical sorting (numeric)

1. Ash      4. Date    7. Grape
2. Beech    5. Elm     8. Hawthorn
3. Cedar    6. Fir     9. Ivy 

Vertical sorting is the default behavior of many of the .Net web multi-column list controls (such as CheckBoxList).

I researched the topic looking at the usual document formatting standards that I used in college (APA, MLA, Chicago), but I have found nothing related to multi-column lists.

Are any actual arguments or document format standards/guidelines/rules for specifying the sort direction of multi-column lists?

I was at a UI-design talk where the speaker suggested that we put more space between things you want the user to see as separate from each other, and less space between things you want the user to see as belonging together.

Since there's a much bigger margin between any two columns than there is between any two lines of text, the brain will see this as three separate columns. The contents of each column belong together into one logical group, and there are three groups. So your second example is much easier to follow -- the first reaction is to scan through (down) the first group (column) before jumping all the way over to the next group.

If there weren't those big gaps between columns -- e.g., if you had a fixed-width font and every label was the same number of characters, and you had only one space between each column instead of a larger margin -- then yes, people would read across with no trouble. But the instinct to group things is stronger than the learned behavior of reading left-to-right.

I personally would have thought right-to-left, top-to-bottom "reading order" would've made the most sense - however, I'm starting to wonder about that in this situation.

I guess the easier to 'scan' with the eyes may well be vertical as it's much easier to scan the first few characters of left-aligned text than scanning across and looking at the first characters of columnized text.