Also called: atd angle · Axial triradius · t-triradius · Triradius · Digital triradii a, b, c, d

Where on the hand: The axial triradius (t) sits low in the palm near the wrist, usually on the midline above the rascettes; the digital triradii a, b, c and d sit just below the bases of the index, middle, ring and little fingers respectively. The atd angle is the angle at t formed by drawing lines from the a-triradius (under the index) and the d-triradius (under the little finger).

What it looks like: A triradius is a tiny Y — the point where three fields of skin ridges running in different directions meet. It is a few millimetres across and is visible only in a good print or under magnification.

This is the one part of palm-reading vocabulary that is also standard scientific dermatoglyphics. In healthy adults the atd angle usually measures between about 30° and 65°; the more distally (higher toward the fingers) the axial triradius sits, the wider the angle. Hand analysts use the triradii to confirm that a loop or whorl is a genuine pattern and to locate the boundaries of the mounts objectively, rather than by eye.

Read with care: Medical dermatoglyphics associates an unusually wide atd angle (over about 57°) with trisomy 21 and studies it statistically in populations — it is a research correlation, never a diagnosis from a single hand. Claims circulating in popular hand analysis that a narrow angle indicates higher intelligence are not supported; the sources that report the correlation say explicitly that the angle cannot predict IQ in an individual. Present the atd angle as a measurement technique, not as a fortune-telling feature. The 57° 'trisomy-21 threshold' sometimes quoted is not usable: it falls inside the stated normal range of 30–65°, so it cannot discriminate in an individual. The group difference is real (Down syndrome mean ~81° vs ~48°); the cut-off is not.