antiques markings!!

[BobbyBobBob]

valuable antiques have certain markings on them from the maker, they also have marking to show when they were made. I was recently looking at an antique by Nathaniel Mills, hallmark 1826, to show this it had a letter 'C' on the underside. Apparently threres different letters for different dates. I am interested to know what these letters actually mean and how they are decided upon. If anybody could tell me it would be greatly appreciated. THANX!!

[jjcaa1995]

Year letters in top angle of diamond A-1845 B-1858 C-1844 D-1852 E-1855 F-1847 G-1863 H-1843 I-1846 J-1854 k-1857 l-1856 M-1859 N-1864 O-1862 P-1851 Q-1866 R-1861 S-1849 T-1867 U-1848 V-1850 W-1865 X-1842 Y-1853 Z-1860 Year letters in right-hand angle of diamond A-1871 C-1870 D-1878 E-1881 F-1873 H-1869 I-1872 J-1880 K-1883 L-1882 P-1877 S-1875 U-1874 V-1876 W-1878 X-1868 Y-1879 Bobby never herd of Nathaniel Mills ( Donald Mills and Henry Mills yes )