The sublingual bar can be used for mandibular removable partial dentures; where the  height of the ﬂoor of the mouth in relation to the free gingival margins is:
MANDIBULAR SUBLINGUAL BAR
Indications for Use: The sublingual bar should be used for mandibular removable partial dentures when the height of the floor of the mouth in relation to the free gingival margins will be less than 6 mm. It also may be indicated whenever it is desirable to keep the free gingival margins of the remaining anterior teeth exposed and depth of the floor of the mouth is inadequate to place a lingual bar.
Contraindications for Use: Remaining natural anterior teeth severely tilted toward the lingual.
Characteristics and Location: The sublingual bar is essentially the same half-pear shape as a lingual bar, except that the bulkiest portion is located to the lingual and the tapered portion is toward the labial. The superior border of the bar should be at least 3 mm from the free gingival margin of the teeth. The inferior border is located at the height of the alveolar lingual sulcus when the patient’s tongue is slightly elevated. This necessitates a functional impression of the lingual vestibule to accurately register the height of the vestibule.
Key Concept:
The sublingual bar should be used for mandibular removable partial dentures when the height of the floor of the mouth in relation to the free gingival margins will be less than 6 mm.
Ref: McCrackens Removable Partial Denture Ed 13th Pg 53
Less than 6 mm
Greater than 6 mm
Less than 8 mm
More than 8 mm