CONDITIONS obtaining in the lumbo-sacral region are becoming of more and more importance. Many disabilities, formerly classified as lumbago, back strains, etc., are now being recognized as due to displacements, either permanent or temporary, of the fifth lumbar vertebra from its normal relationship to the sacrum, or to strain primarily due to structural weakness in this region. Displacements of the fifth lumbar, when fairly marked, are easily recognized either by antero-posterior or lateral roentgenograms, but there are numerous instances where distinct subjective symptoms are present but the roentgenologist's report is negative. We are not now in position to say whether certain positions of the lumbo-sacral joint are within normal limits or not. That is, to say that the position seen cannot be responsible for the symptoms, for variation in symptomless individuals is common. It is very necessary, therefore, that we have a fixed average norm, a deviation from which constitutes a definite abnormality. The object of this report is to suggest observation of a large number of symptomless individuals, as well as those with symptoms, to determine how far from a definite sacral base line, here described, the fifth lumbar vertebra may be placed and still be within the norm. If this can be established it is believed that many back strains, which are now reported as negative roentgeno-graphically, will be found due to comparatively slight displacements, anterior or posterior, beyond a definite average norm. The base line suggested by the author is established on a lateral view of the lumbo-sacral region as follows: So far as can be determined at present, it is unimportant whether the tube be centered over the joint or over the third or fourth lumbar vertebra. Neither is it necessary to have a true lateral. Two points are taken on the edge of the sacral articular surface, as far apart as practicable. These points are then connected by a straight line which is projected forward until it intersects a line which is either the anterior margin of the sacrum or a prolongation of it. If the anterior-superior margin of the sacrum is an angle the articular surface line will intersect the anterior surface line at the point of the angle. If rounded, both lines must be prolonged until they intersect. A line at 90 degrees to the articular surface line is now erected at this point of intersection. It is believed that this line, perpendicular to the sacral articular surface, should pass well in front of the anterior-inferior margin of the fifth lumbar unless there is an anterior displacement of this vertebra. No symptomless individuals have so far been found where this is not true, and in one instance where the fifth lumbar was just short of touching the line, there were marked symptoms and considerable disability. In a patient of Dr. W. B. Bowman's, with distinct anterior displacement, the line cut through the body of the fifth lumbar.