The letters printed on a conductor are a code for what its insulation can take -- heat, moisture, the conditions it is rated for.
People learn to recognise a conductor by the colour of its jacket and the reel it came off. Neither of those is the rating, and two conductors that look the same can be rated for different places.
THHN, THWN and XHHW are different designations and they mean different things about the insulation. Reading them is the start, not the end: the complete marking and the listing are what actually establish what the conductor is approved for, and the location you are installing in is what decides whether that is the right conductor.
Check yourself: A conductor has a white jacket like one you used in a wet location last week. Is it rated for a wet location?
Answer: No. Do not assume a wet-location rating from the jacket colour -- read the listed designation.
Colour is not a rating. The designation and the complete marking are what state the insulation characteristics, and the listing is what backs them.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.
Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.
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