Use this procedure to add lines to a grid with evenly spaced lines. The lines will always be distributed evenly along the grid axis. If you need non-regular lines with different spacing between them, open the grid’s Properties palette, and change the Layout type to Manual.
- Select the ceiling grid to which you want to add lines.
- Add grid lines:
If you want to… Then… add grid lines to the X axis of the grid click 
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Then, click on the X axis of the grid. The number of grid lines increases by one, while the overall size of the grid remains the same. add grid lines to the Y axis of the grid click . 
Then, click on the Y axis of the grid. The number of grid lines increases by one, while the overall size of the grid remains the same.
Your grid has been updated accordingly, with the new number of grid lines distributed evenly along the grid axis.