The 1820 date is among the most curious and interesting of the many fascinating half eagles of the 1820s, if not on a par with the fabulously rare 1822, of which but three examples are known, or the 1825/4, which only has two pieces known. Going by the rarity estimates given in Bass-Dannreuther, from 375 to 476 examples of half eagles from 1820 through 1829 are known, of which the 1820-dated varieties constitute about one-third of the total. (The recorded mintage of 1820 half eagles, at 263,806 coins, was an overlarge one that was never again even approached during the 1820s.) However, since there are nine varieties of 1820 half eagle, all of them individually range from 'rare' (R.5) to 'extremely rare' (R.7). In fact, of the 25 known die pairings of half eagles dated from 1820 through 1829, one only single die pairing, the R.4 1823 (BD-1, the only known marriage for the year) can be considered less than rare, with an estimated 80 to 100 known survivors.       

Focusing on only the varieties dated 1820, the 2012 Guide Book lists three varieties for the year: the Curved Base 2, Large Letters, the Curved Base 2, Small Letters, and the Square Base 2 (all of which are Large Letters marriages, there are no known Square Base 2, Small Letters variants). Bass-Dannreuther lists four varieties with Square Base 2, Large Letters: BD-1, R.7, BD-2, High R.5, BD-3, R.5, and BD-4, High R.7. There are two Curved Base 2, Large Letters varieties, BD-5, R.6, and BD-7, High R.7, and there are three Curved Base 2, Small Letters varieties: BD-6, R.7, BD-8, High R.7, and BD-9, High R.7.