X Gantry Leveling/Alignment Blocks
Having your X Gantry level is important for quality prints and especially if you are using an ABL probe (like the EZABL). You want your gantry level with the printer frame, not your bed. Your bed should then be leveled to the nozzle AFTER you are sure your X gantry is level with the printer frame.
Print out 2 of these blocks and make sure they are within 1mm height of eachother. You can use them to help get your gantry aligned.
Example pictures of using the blocks to check an Ender 3 Gantry alignment/leveling.