A dental x-ray is flat. It can prove something's there — an extra root, a stray fragment — but never whether it sits toward the cheek or the tongue. One elegant trick always tells you. It's yours by the end of this page.
~6 min9 cards★ ≈8% of the RHS exam
▶ Watch · 1:54
Watch first — the SLOB rule in under two minutes.
1The problem
Flatten a 3-D mouth into one picture, and you lose a dimension.
A single radiograph shows a mystery object floating near a tooth. It's obviously there — but the film can't say whether it's near the buccal · cheek side or the lingual · tongue side. One flat image simply doesn't carry that information.
So we do something clever: take a second image from a slightly different angle, and let the object give itself away by how it appears to move.
2Try it yourself
Shift the tube. Read the film.
Choose where the object actually sits, then slide the tube. Watch the bright dot on the film — and read what it's telling you.
Tube-shift localizer
Interactive
Cross-section — where things really are
The film — what you'd actually see
tube lefttube right
Slide the tube and watch the dot.
3The rule, in four letters
Same direction → Lingual Opposite direction → Buccal
If the object drifts the same way the tube moved, it's on the tongue side. If it runs the opposite way, it's on the cheek side. Say "SLOB" once and it's permanent.
4Why it works
You already know this — it's just parallax.
Hold up two fingers, one near, one far, lined up. Slide your head sideways: the far finger drifts with you; the near finger jumps against you.
On the film, the lingual object is the far one (moves with the tube — same); the buccal object is the near one (moves against — opposite). That's the entire secret.
5Now the real words
Same idea, dental vocabulary.
Everything in the finger analogy has a dental name — here's the translation, since these are the exact words the exam uses:
In plain EnglishOn the exam
👉The far finger
→
LingualToward the tongue. On the second film, a lingual object moves the same direction as the tube shifted.moves with your head — same direction
☝️The near finger
→
BuccalToward the cheek. On the second film, a buccal object moves the opposite direction from the tube shift.moves against your head — opposite direction
↔️Moving your head sideways
→
Shifting the PIDPosition Indicating Device — the cone-shaped tube that points the beam. You aim it from a different horizontal angle for the second exposure. between two exposuresthe "tube-shift" step
🔤This whole trick
→
SLOB ruleSame Lingual, Opposite Buccal. Also called Clark's rule, the buccal object rule, or the tube-shift technique — four names, one method. · Clark's rule · buccal object rule · tube-shift techniquefour names for one method — the exam uses all of them
6Spot it on the exam
DANB describes the shift — you name the side.
"Object moved the same direction as the tube shift"
→
Lingual — the far object, tongue side
"Object moved the opposite direction from the tube shift"
All mean SLOB localization — same method, four names
7Check yourself
On the second image, the object moves the same direction as the tube head. Where is it?
Lingual. Same → Lingual. It moved with the tube, so it's the "far finger."
7Check yourself
"Clark's rule," the "buccal object rule," and "tube-shift" all name the same thing. What is it?
SLOB localization. Watch for the synonyms — they're all one method with four names.
Recap
The whole trick in 5 lines.
✓One flat image can't tell buccal from lingual — you need a second image from a different angle.
✓Same direction as the tube shift → Lingual (far object, moves with you).
✓Opposite direction from the tube shift → Buccal (near object, moves against you).
✓It's just parallax — the same effect as two fingers held at different distances.
✓Clark's rule, the buccal object rule, tube-shift technique, and SLOB are all the same method.
That's the whole trick. On test day, you won't reason it out — you'll just know.
When a question describes a tube shift and which way the object moved, you'll read the answer straight off four letters. That's mastery, not memorization.
