The locations that can be protected by voiders are the shoulders, elbows, abdomen, and knees. A voider costs and weighs 1/5th of the cost and weight
Example: A knight in head to toe DR7 medium plate wants to add voiders to his armor. He can wear up to heavy armor without taking a DX penalty. Adding DR3 thin mail voiders would not increase the bulk at all, while adding DR5 light mail voiders would increase it by one step to heavy. If he wore DR7 medium mail voiders under his medium plate, the bulk would increase two steps to jousting and he'd have a -1 penalty to DX.
Example 2: A SM-6 pixie in DR5 light orichalcum plate counts as wearing thin armor, which does provide a DX penalty. Adding DR3 thin mail voiders would not increase the armor bulk, but adding DR5 light mail voiders would. Any pixie rich enough to afford light orichalcum plate can probably afford DR5 light mithril mail voiders, which are effectively thin for bulk purposes and can be safely worn under the orichalcum plate.Voiders are normally attached to clothing, such as an arming doublet or gambeson, that is worn under the rigid armor. However, voiders can just as easily be attached to the rigid armor itself. Voiders can also be worn under flexible armor, though there is normally little point since flexible armor doesn't have chinks.
For easy reference, voiders for all the joints cost and weigh 5% of a suit of armor of equivalent DR.
Sample VoidersLeather reinforcement: Leather strips intended to be worn beneath scale armor. DR2 leather elbow and shoulder voiders that increase armor bulk by one step. Costs and weighs 2% of the base armor (DR2 Leather is $120 and 26 lbs) for $2.2 and 0.5 lbs.
Heroic Arming Double: Shirt and hose reinforced with strong mithril mail at the joints. Full coverage DR5 very fine, mithril mail voiders that do not increase armor bulk. Costs and weighs 5% of the the base armor (DR5 very fine mithril mail is $110000 and 15 lbs) for $5500 and 0.75 lbs.