The Abacus is an accessory that adds 3 points to your Engineering skill when equipped in your Accessories bar. You can find it in the Accessories section of your inventory when it is not equipped. Like the T-Square, it is the only accessory for its skill. In other words, there are no lower-level versions of the Abacus to raise Engineering by only 1 or 2 points.
How to Obtain
You can purchase this item for crowns from the "Tools And Accessories" section of the crown store, where it is only available as part of the Engineering Pack (which also includes the T-Square). You can also try trading for it in a market world.
How to Use
To equip this item, drag it into an active slot in your Accessories bar.
The Abacus was added in 2013. For a short time, it was possible to loot it.