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X-ray examination is the first opportunity a dentist has to evaluate a tooth's root canal system.
In this example, we're looking at a lower 1st molar.
It's easy to see that the root labeled "b" has two distinct canals.
However, lower on down it's hard to ascertain whether they coalesce into a single canal or remain separate.
This tooth root is well known for having two distinct canals, so it's probably the latter case.
The root labeled "a" appears to have a large single canal, which is the usual configuration.
However, on an x-ray two separate canals (a common variant for this root) may be overlapped and give the impression of just a single one.
Taking a second x-ray, at a different angle, can help a dentist make an accurate determination.