In Maya geometry has a history. This history describes what has happend to that geometry, and how it was made. This information can be used to your advantage, because you can change things (for example values) afterwards. Which results in a more interactive way of modelling. For example; if you make an object based on curves, and change these curves, your object will also change (see image below).
History can be recognized in several ways. The most obvious is in the channelbox on the right side of your screen. This contains the regular X, Y, and Z transformation values, but also inputs and sometimes outputs. If there is information in the in- or outputs box, there is history. You will see outputs if your selected piece of geometry is used to make another piece of geometry.
Also history is recognizable if you select an object, and the output geometry becomes purple (in wireframe mode).
In order to change an object with the use of history, you can do several things:
- Change the object(s) that were used to make the with history connected object (as done in the above example).
- Change the value in the channelbox numerically, by clicking on an out- or input connection.
- Use the manipulator tool to change the value graphically (as done in below example).
- Deleting history
History is often very handy, and it is recommended to keep it as long as possible. But sometimes it gives unwanted results. For example, when you select some objects and move them, some objects move twice as fast. This is related to the history of these objects, where it accumulates the movement.
If you want to prevent this from happening, you can either select only the output geometry, and not the objects that were used to create the output geometry (like the curves in the above vase example), or delete the history. Which can be found in EDIT - DELETE BY TYPE - HISTORY.
Once the history is deleted, you cannot get it back. The geometry becomes dumb, and is no longer linked to other objects, and you will no longer be able to take advantage of the link between objects.