The new 12-sided £1 coin was introduced into circulation in the United Kingdom on 28th March 2017, with some of the coins minted earlier and dated 2016.
The coin has a 12-edged shape, similar to the pre-decimal brass threepence coin; it has roughly the same size as the previous £1 coin, and is bi-metallic like most £2 coins. Bimetallic means it is made of two metals - the outer ring is gold coloured (nickel-brass) and the inner ring is silver coloured (nickel-plated non-ferrous alloy).
A security feature is a "latent image" - something like a hologram that changes from a ‘£’ symbol to the number '1' when the coin is seen from different angles. Another security feature is micro-lettering - very small lettering on the lower inside rim on both sides of the coin, "One pound" on the obverse (“heads”) side and the year of production on the reverse side, for example 2016 or 2017.
The new design is intended to make counterfeiting (which affected the earlier type significantly) more difficult, and also has an undisclosed hidden security feature called "iSIS" (Integrated Secure Identification Systems), thought to be a code embedded in the top layer of metal on the obverse of the coin, visible only under a specific wavelength of ultraviolet light.
The legal tender status of the earlier (round) £1 coin was withdrawn at midnight on Sunday 15th October 2017.