The grid is a coordinate system of "construction" lines that form a framework to assist you in drawing the model. Coordinate systems may be Cartesian or Cylindrical. When a new model is started, grid spacing must be specified as uniform. Any number of grid lines with arbitrary spacing in each grid can be defined. Thereafter grid lines can be added, moved, and deleted, as described herein. The initial grid system is called the Global coordinate system. Any number of additional coordinate systems can be defined.
Drawing operations tend to "snap to" grid line intersections, unless you turn this feature off. This facilitates accurate construction of your model.
Use the Edit menu > Grid Data command to access the following subcommands:
Add/Modify Grids
Form: Coordinate Systems
Use the Edit menu > Grid Data > Add/Modify Grids command display the Coordinate Systems form and make changes to the grid system of your model.
Select the system to be modified and click the button to access the Coordinate System form. In addition to allowing the gird locations to be set -- including skewed grids under the General Grid Data area -- the origin of the coordinate system may be shifted for coordinate systems other than GLOBAL. This form also allows specification of the bubble size and if grid lines are shown or hidden, and, if working with the GLOBAL system, the model datum and story heights.
Add Grid at Selected Points
Form: Add Grid Lines at Selected Points
Use the Edit menu > Grid Data > Add Grid at Selected Points command to add a grid line(s) at a selected point.
Select the point where the grid line(s) is to be added. The 
button or Select menu > Select command enables the Select mode.
Click the Edit menu > Grid Data > Add Grid at Selected Points command to access the Add Grid Lines at Selected Points form
Select the Coordinate System from the drop-down list.
Check the Parallel to X check box, Parallel to Y check box, or the At Angle(s) From X check box, or any combination of the three.
If the At Angle(s) From X check box is marked, specify the angle that the grid should be rotated off of the X-axis. Entering more than one angle separated by a semicolon will result in a grid line being drawn for each angle.
Glue Points to Grids
The Edit menu > Grid Data > Glue Points to Grids command "glues" joints to the grid line so that when the grid line is moved, the joints move too. This is a powerful way to quickly edit the model without redefining the joint locations. The command is a toggle and is enabled when a check mark precedes the command.
Lock On Screen Grid System Edit
Use the Edit menu > Grid Data > Lock On Screen Grid System Edit command to toggle on screen editing of grids off and on. This lock applies only to Cartesian coordinate systems. This lock is active when a check mark is adjacent to the menu command.
If unlocked, a grid line can be moved graphically by clicking the Draw menu > Reshape Object command and then selecting the grid line to be moved. Grab either the handle at the grid line end to move it relative to the other end, or grab the grid line between the ends to move it parallel to its original location.
Also see