Guards must be constructed so that people cannot fall through. This is particularly important where young children may be expected to test railings. In some jurisdictions, spindles (also called balusters) must be designed so that no opening will allow a four inch ball to pass through. In other jurisdictions, the ball size is six inches. Check with your local authorities to determine what is required in your area. Again, you can recommend something different, but you must know what the minimum standard is for new work. Spindles should be substantially vertical. Railings with many horizontal details are easily climbed by children and, as such, are a safety hazard. These are not allowed by some authorities.
These are design issues. Custom-designed railings are often the worst offenders from a safety standpoint with respect to spindle spacing and climbability.Use common sense when looking at guards and think about toddlers and small children who love to climb.
Some people refer to spindles in railings as balusters. The entire railing system may be called a balustrade. Freestanding posts at the ends of railings (particularly at steps) are often called newel posts.