Mouth quite level, but of the two a slight overshot mouth is preferable to an undershot one; teeth are somewhat smaller than would be expected in a Terrier of the size.

1901
Lionel Jacob

Mouth quite level, but of the two a slightly overshot mouth is preferable to an undershot one. The teeth are somewhat smaller than would be expected in a terrier of the size. In this respect the breed seems to suffer to an extraordinary degree from cankered teeth. I have never yet seen an imported specimen with a sound mouth.

The jaws are generally slightly “undershot”; some times there are very much

1935
Standard

Mouth level otherwise slightly undershot preferable.

Mouth : Level. Otherwise slightly undershot preferable. An overshot mouth is undesirable. Muzzle of medium length; a square muzzle is objectionable.
A black beard is an asset.

1961
Dr Guy LESCURE
Vétérinaire

The jaw has a trend to present a small undershot
The inferior maxillary has a trapezoidal form, the lower jaw may meet the extreme of the upper jaw in such a way that incisors join exactly, but it may happen that it can be a little bit longer and so creates a slight inferior prognathism . In the subjects observed, this undershot has always been lower than 1/2 cm.

Palate is black, very strongly pigmented.

"scissors bite": The mouth being closed, the inner part of the upper incisors should close and cover the outside part of the lower incisors on approximately one fourth of its coronary portion.

“level” - the upper and lower incisors should close just end to end.

“Prognathism” : Greec “pro’ (for ahead), and “gnathos” ( for jaw).
Defectuosity very common in canine breeds.

”undershot” - Lower prognathism - When the lower incisors close just outside the upper incisors . The “true” prognathism is due to a shortening of the upper jaw compared to the lower mandible and not to a lengthening of the lower jaw.

“overshot” – Upper prognathism – The upper incisors exceed
the lower incisors.

In both cases there is an interval between the incisors of the two jaws.

If this interval is simply due to a too oblique implantation of the teeth, then it is not
a true prognatism, it is just a minor defaut , of minor importance.

The “t rue” prognatism concerns all the jaw and is not affected by
a bad implantation of the teeth. It is only a bad conformity of the bones
which determine a mandible prognathism.

Mouth : Upper incisors should close just inside the lower, i.e., a reverse scissor bite. Incisors should be nearly in a straight line. Full dentition is desirable.

The preferred bite is either level or slightly undershot.

The bite is reverse scissors (upper incisors just touching the inner face of the lower incisors). Full dentition. Incisors (6) to be in a straight line. 
Acceptable bite: level (the front incisors of the upper and lower jaw meeting edge to edge).
Undesirable bite: overshot, excessively undershot (more than 1/8 in. (.3cm). The teeth must not show when the mouth is closed.

Upper incisors should close just inside the lower - this is a reverse scissor bite.  Incisors (teeth across the front) should be nearly in a straight line.  Full dentition is desirable.

Upper incisors close just inside lower, i.e. reverse scissor bite.  Incisors in as broad and as straight a line as possible.  Full dentition desirable.

« Incisors in a broad and straight a line as possible » would do justice to a Shi Tzu mouth !

It has always been noticeable that in the Lhasa Apso, the four middle incisors in the lower jaw are straight with the two outers ones set fractionally back – and the same with the upper jaw; all in a straight line usually brings with it a bite that is too undershot, with prominent “eye” incisors in the top jaw, tending to be more to a Shi Tzu bite.

The Lhasa Apso Club - 30th Anniversary - 1957 -1987

Mouth :: Upper incisors close just behind lower, i.e. reverse scissor bite. Incisors in a broad and as straight a line as possible. Full dentition desirable.

Face		Profil