Secondary Navigation Belongs on the Left
Similar to how your primary navigation should be on the top of your page, your visitors will expect your secondary navigation to be on the left of your page. There are other options to make this intuitive, especially other things to consider when building a navigational structure that is more than two levels. However, placing your secondary navigation is always a solid choice.
If you’ve ever tried to pay attention to your browsing habits while on a site with the navigation on the right, you’ve probably experienced the frustration of needing to remember to look at the right side to navigate and orient yourself as you browse. Not only is it not typical, it’s not intuitive.
Again, since we read left to right, and top to bottom, we look to the top and to the left to orient ourselves and when we are looking to further navigate. Placing side navigation under the primary navigation helps your visitors associate them together as navigation. Once in a sub-section, this new menu on the side attracts attention while a visitor is in navigation mode.
Secondary Navigation Can be Longer
It’s okay to use more items in a vertical menu because vertical menus are easier to read. Target between 10 and 15 items in a left-hand secondary navigation.
Because side navigation is stacked vertically, it is easier to scan than horizontal navigation. This means that (within reason), you can place more items in your secondary navigation than would be ideal for for a top-level navigation without impeding your visitor’s scanning. Your visitors can scan two items at a time when stacked vertically, so 10-15 items here is acceptable when needed.