difficult to delineate; especially when the patterns of both headlights overlap each other. Using these tools first helps insure the two patterns are very close to their proper alignments. From there it is easy to discern the pattern edges and make the final precise adjustments. If you are just bumping the light pattern up or down a few inches then these tools are not needed. However, if the body was tweaked in an accident, or other major changes were made that affect headlight alignment; then the aiming tools can get you back to dead center very quickly and accurately.
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