https://www.runjeetsingh.com/antiques/artworks/theta-armour

This long set of armour with ‘theta’ mail links is rare, the links so-called on account of the fact that each ring has a bar fused across its centre, so that they resemble the Greek letter ‘theta’ (or ‘θ’). This imbues the armour with added strength, though at the cost of considerably greater manufacturing time and expense.

Why is Trump wearing mail armor?