In WordPress permalink structures control how the URL strings will be formed for the blog section of your site. By default the string appears more as code and not as an intelligible sentence. I strongly prefer it to appear as a readable sentence so users can understand it more easily and because search engines strongly consider them for search queries.
WordPress can be a little temperamental with how it stores and retrieves your blogs, which are organized in a mySQL database. If the first URL section is not numerical your site can experience major downloads, though it isn’t very likely until you have a large amount of posts.
The permalink structure I prefer most as it begins numerically by also containing valuable data is as follows:
This can be broken down into its three sections separate by the slashes.
The year is just that and for some blogs that have very frequently submissions you can also add %monthnum% before or after the year, providing even more data to users and search engines. %monthnum% will display the number of the month that article or blog is published.
Category is self-explanatory. Postname is the name or title given to the article. Note that you can manually change the Postname in any blog, which is located directly below the title and has an edit button to its right.
%post_id% vs %year%
Some prefer to use %post_id% instead of %year%, which displays the number assigned to the post. However, I prefer %year% in most cases as it has some actual meaning and is not just randomly assigned.
Penalty on long URL strings
Note that Google does begin decreasing the value of words in strings longer than 3 words and likely penalizes words further down the string. This doesn’t hurt the value of the string, but rather just the words further down the string. Still this is very important to consider.
Another way WordPress works, and I’ve never entirely understood why, is that if you check only the sub-category then your URL string will display both the parent and the child category; However, if you check both the parent and the child category then only the parent will display. I don’t know why this is and if anyone else does I’d love to hear it.
For a complete list of Permalink options in WordPress check out the Codex.