Above 36 Inch Height Maximum Spacing
Pay attention here, because this building code has changed.
You "were" allowed to have an 8 inch spacing in which an 8 inch sphere was allowed to pass through the area above 36 inches on a guard rail, but this is no longer the case.
Reference: 2012 International Building Code - 1013.4 page 260
There is an exception under the opening limitations that says you
shall not be able to pass a round sphere greater than 4 3/8 inches through
the small section of a guard rail in between the 36 inch height and 42 inch
In other words below 36 inches the maximum distance in between any part of the stairway guard rail parts or components like the banisters and balusters should not be larger than 4 inches and in between the 36 and 42 inch measurement, it switches to 4 3/8 inches.
The picture above is a good example of what I'm talking about. Remember, this isn't a common method used for constructing handrails and you should always check with your local Building and Safety Department, before designing or building anything.