A half-sovereign was current at 10 shillings. This is the first Half Sovereign issue for George IV (1820-1830).
The reform of 1817 terminated all guinea-denominated issues in favor of the sovereign. This one year type is rare as it was withdrawn almost as soon as issued, on account of its likeness to the contemporary sixpence. Unlike the later issues, it features a heavily garnished shield on the reverse which is rounded, whereas the later ones were angular.
From Whitman's "Guidebook of English Coins, Nineteenth and Twentieth Centuries":
"The half sovereign of 1821 has the laureated head of the King designed by Pistrucci. The reverse side has an oval shield garnished with roses, thistle and shamrock. Although designed by Merlen, the letters WWP (for William W. Pole, Master of the Mint) are in the centers of the shamrock leaves.
This half sovereign was quite similar in size and design to the sixpence, and as a consequence many of the sixpences were gilded and passed as gold pieces. To stop this practice the issue was withdrawn, which accounts for their scarcity."