Quarter of an A4... or "Postcard Size"

A6 is postcard size. It is roughly one quarter the area of an A4 sheet. Page sizes are part of the ISO 216 standard. A readable guide to the rationale behind the design is available here.

What is Tantalum? A chemical element used in Capacitors.

Tantalum is a chemical element whose name comes from the Greek villain Tantalus.

Tantalus was reputed to live in Tartarus, the deepest portion of the underworld.  He was seen there by the legendary Greek hero of Homer's Odyssey.

Tantalum is a rare, blue-grey transition metal, highly resistant to corrosion. Its main use is in capacitors for computers and mobile phones.

It was first discovered by a Swedish chemist, Anders Ekeberg, in 1802. A trade association (TIC) was formed in 1974 based in Belgium for the tantalum and niobium industries.