Is this T-shirt true to size?

The listing mixes loose and slim fits, so check the specific size chart before ordering. If you are between sizes and want a relaxed look, size up; for a close fit under a jacket, take your usual size.

Can I wear it under a blazer?

Yes. The mock neck and solid black make it a clean substitute for a shirt under a blazer or jacket. Keep the collar visible and skip a tie.

Is it warm enough for cold weather?

It is a cotton jersey tee, not a knit sweater. The high collar helps at the neck, but in cold weather wear it as a base layer under a jacket or overshirt.

How should I wash it?

Machine wash cold and lay flat or hang to dry to protect the ribbed collar and keep the cotton from shrinking. Avoid high heat in the dryer.

Is it a turtleneck or a mock neck?

It is a mock neck: the collar sits high on the neck but does not fold over like a turtleneck. The listing also calls it a mandarin collar, which describes the same stand-up shape.