The brass string level is used to bring the strike point of grand strings, both bass and treble, into level. The tool is 2 oz of brass, 3/8 inch wide x 1 1/4 inch tall. This allows the tool to be used on most grands very near the strike point just in front of the dampers. 
 
To  check string leveling, place the tool on the string unison with the foot of the tool across and touching all strings of the unison. Then, with the dampers raised, pluck each unison. The difference in sound will reveal an open string. Watching the bubble will show if the strings are in a level plane. If the level rocks back and forth the middle string is too high.
 
Level strings and correctly mated strings can eliminate many voicing problems.