How much minimum amount of vertical space should be there between the floor of the mouth and gingival margins of teeth, to use a lingual bar?

A Lingual bar major connector should be located at least 4 mm inferior to gingival margins and farther if possible. The vertical height of a finished lingual bar should be at least 4 mm for strength and rigidity (So total 8 mm space is minimum requirement between gingival margins and movable floor of mouth). If less than 8 mm exists between gingival margins and the movable floor of the mouth, a linguoplate, a sublingual bar (when the height of the floor of the mouth in relation to the free gingival margins will be less than 6 mm.) or a continuous bar is preferred as a major connector. Key Concept: A Lingual bar major connector should be located at least 4 mm inferior to gingival margins and farther if possible. The vertical height of a finished lingual bar should be at least 4 mm for strength and rigidity (So total 8 mm space is minimum requirement between gingival margins and movable floor of mouth). Ref: McCrackens Removable Partial Denture Ed 13th Pg 53