Bottom up/top down (or top down/bottom up, the order makes no difference) blinds are different from normal window blinds. Normal window blinds have a single mechanism at the top of the window that allows the blinds to pull up and down (vertical window blinds). Bottom up/top down blinds, as the name might suggest, allows the window blinds to function in a similar manner. However, while the blinds may be pulled up from the bottom of the window to let in light, they may also be pulled down from the top to expose the upper portion of the window while keeping the bottom portion covered.
Bottom up/top down blinds actually function as two separate sets of window blinds. There is a rail in the bottom of the blinds and a rail in the middle of the window. Each rail also has its own pull cord. The pull cord on the right side of the window controls the bottom rail (and raising the bottom of the blinds up to the center of the window), while the pull cord on the left side operates the middle rail (pulling the top of the blinds down to the middle of the window).
Additionally, the entirety of a set of bottom up/top down blinds can be raised to the top of a window if the middle rail is raised to the top, and then the bottom rail is also raised up afterwards.
Some bottom up/top down blinds can be purchased with an automated control. This option is more expensive, since small motors will drive both of the rails. However, there are no cords to bother with, and the ideal amount of window can be revealed with the push of a few buttons on a remote control.
The extra cost of an automated set of bottom up/top down blinds may also be worth it if a person has small children or pets. In both cases the cords could be a choking hazard, or they could become endlessly tangled because they became a play toy.