Practice chest
The Practice Chest is a chest found in the Thieving guild, and require the completion of From Tiny Acorns. There are six practice chest in total. There are two types of practice chest, 3 regular practice chests on the north side of the room and 3 complex practice chests on the south side of the room. Each practice chest is contained in its own jail cell, and jail cell doors needed to be opened to access the chests, giving additional experience for doing so. Doors will stay open for several minutes.
Regular practice chests, on the north side of the room, require level 26 Thieving to be opened and can be opened for 30 Thieving XP and one Hanky Point (a single blue hankerchief). They may take a little time to open, but it is recommended to be patient and keep trying. The doors to the jail cells containing these chests give 210 XP for opening. They also require level 26 Thieving to open and do not require a lock pick to open.
Complex practice chests, on the south side of the room, can be opened for 180 Thieving XP and four Hanky Point (a single red hankerchief). The doors to the jail cells containing these chests give 280 XP for opening. Level 35 thieving is required to open both the chest and the cell door. A lockpick is also required to open both door and chest. If the door closes while you are inside the jail cell, and you open it, you will not gain experience.
Due to the increased xp and hanky points, the complex chests to the south are considerably more useful when training.
Training
When training on the chests, it is recommended to use the complex chests. To start, open the complex door closest to the hankies on the wall, and open the three complex doors in that order going outward. Then, open the regular doors outward going in, as doors give more experience. This is the order you will open doors every time they close. (note: if trapped in a cell when the door closes, start with that cell when opening, and you'll just start there from now on). Once your doors are open, starting at the complex chest on the wall where you started, open the three complex chests. At lower levels, by the time this is done, your last chest is opened the first will have reset, and you can begin again. At higher levels (55 or so and up), you will need to open up a few normal chests to keep the xp going while waiting for complex chests to reset.