Customize menu > Grid and Snap Settings > Home Grid tab
Prompt line > Right-click 3D snap toggle.
The Home Grid panel of the Grid And Snap Settings dialog sets the spacing and other characteristics of the home grid. Choosing useful home grid settings can simplify the construction process. The home grid provides a visual reference to use when creating objects in a scene.
In the software, grids have these primary uses:
An aid in visualizing space, scale, and distance.
Construction planes where you create and align objects in your scene.
A reference system for using grid snaps.
Procedures
To set grid spacing for unit measure:
Choose Customize menu > Grid And Snap Settings from the menu bar.
Click the Home Grid tab. Under Grid Spacing, adjust the value, which is in current units.
For example, if you have units set to centimeters, you might make one grid space equal to 1.000 (one unit, or one centimeter in this case).
To set major grid divisions for multiple units:
Choose Customize menu > Grid And Snap Settings from the menu bar.
Click the Home Grid tab.
Under Major Lines Every Nth, adjust the value, which is the number of grid squares between major lines. The minimum is 2.
For example, if you use a grid spacing of one centimeter, you might use a value of 10 so the major grid divisions represent one decimeter.
In perspective viewports, the displayed size of the home grid adjusts for different major grid divisions. The home grid itself is unchanged.
To set view update options:
Choose Customize menu >Grid And Snap Settings from the menu bar.
Click the Home Grid tab.
Under Dynamic Update, choose either Active Viewport (the default) or All Viewports.
To allow subdivision below grid spacing:
Choose Customize menu > Grid And Snap Settings from the menu bar.
Click the Home Grid tab.
Turn off Inhibit Grid Subdivision Below Grid Spacing.
When you turn off this box, you can zoom indefinitely "deep" into any plane of the home grid. Each grid square subdivides into the same number of smaller grid spaces.
Interface
Grid spacing: Grid spacing is the size of the grid's smallest square. Use this spinner to adjust the spacing (which is in current units), or enter the value directly.
For example, if you have units set to centimeters, you might make one grid space equal to 1.000 (one unit, or one centimeter in this case).
Major Lines every Nth: The home grid displays heavier or "major" lines to mark groups of grid squares. Use spinner to adjust the value, which is the number of grid squares between major lines, or you can enter the value directly, the minimum is 2.
For example, if you use a grid spacing of one centimeter, you might use a value of 10 so the major grid divisions represent one decimeter.
Inhibit Grid Subdivision Below Grid Spacing: Causes gmax to treat the grid as a fixed set of lines when you zoom in on the home grid. In effect, the grid stops at the grid space setting. If you keep zooming, the fixed grid is lost from view. Zooming out is not affected. When you zoom out, the home grid expands indefinitely to maintain the major grid divisions. Default=on.
When the box is turned off, you can zoom indefinitely into any plane of the home grid. Each grid square subdivides into the same number of smaller grid spaces.
For a grid spacing of one centimeter and a major division of 10, the next level down subdivides into millimeter spaces, and so on.
Dynamic Update: By default, only the active viewport updates as you change values for Grid Spacing and Major Lines every Nth. The other viewports update after you have completed changing the values. Choose All Viewports to have all viewports update as you change the values.