There are two main internal compartments for the bag. The smaller compartment is on the top third of the bag and the larger one is on the bottom two thirds. These two compartments are separated by a wall. If this wall was permanent, the lower compartment would be very limited in the length of the gear you could carry and the bag could not be collapsed so that it could take less space when empty. Happily this wall is held in place with velcro on three of its four sides and if needed the wall can be shifted out of place, so that you could collapse the bag a bit and you could internally carry something longer than the bottom compartment. Please note, when the wall is shifted out of place the top compartment no longer exists as a separate entity and thus becomes a continuation of the bottom compartment. Assuming that this wall is not shifted out of position, the compartment at the top of the bag is accessible by one zipper and the compartment at the bottom can be opened by three different zippers. Each of the zippers has two separate sliders that can meet at any point along the length. Attached to the sliders are tabs made of a short nylon cords terminating with a molded plastic triangle with a hole that allows them to be temporarily secured with a (not included) mini padlock.
The top compartment can carry small items such as a can of soda, a 500ml water bottle, folded headphones, battery chargers, etc. Along the internal walls of the top compartment are a pair of pockets made of a mesh type fabric. One pocket is secured with a zipper and the other is closed with a velcro flap. The zippered pocket is about the size of a passport and the velcro flapped pocket is slightly smaller. The mesh fabric has a bit of stretch to it, so whilst small, these pockets can easily hold larger items. Finally, attached to the inside of the upper compartment is a small plastic lanyard hook that allows you to temporarily attach items such as a keychain.
As far as I am concerned the lower (larger) compartment is the primary selling point of this bag as it has the key design features. As I mentioned in the essay that precluded this review, I am interested in a bag that can allow me to access camera gear without fully removing the bag from my back. Because I wanted to save time when shooting whilst on the move AND I was concerned about preventing theft of the contents. About security, perhaps an explanation is in order. I am of the opinion that typical backpacks seem to offer the person standing behind the wearer (of the bag) all manner of pockets and zippers to be opened and perused without the knowledge of the wearer. The Think Tank Trifecta differs from that type of design, in that there is no access to the internal compartments from the back of the bag. Instead you can only access the contents, thru the top, sides or the front of the bag. And if you position the zipper sliders in an intelligent manner, the Trifecta makes it almost impossible for someone to open the zippered compartments without you noticing it.