Pertinent and relevant are close in meaning, but they are not perfect synonyms.
Pertinent usually means directly related to the exact issue being discussed. It suggests a tighter, more precise connection. If something answers the question at hand or speaks to the core issue, it is pertinent.
Relevant means related and useful, but the connection can be broader. Something can be relevant even if it is not the most central point.
- The witness gave testimony that was pertinent to the case.
- The background data was relevant to the discussion, even though it was not the main evidence.
A simple way to remember the contrast is this: if it is right on point, use pertinent. If it is connected enough to matter, use relevant.
In many everyday situations, relevant is more common. Pertinent often sounds a bit more formal and precise, especially in legal, academic, or professional contexts.