Breaking Down Numbers Before Converting Into Roman Numbers Arabic numbers decomposing (breaking down) in place value subgroups before converting to Roman numerals:
To turn any Arabic number into a Roman numeral, break down that Arabic number (decompose it) into place value subgroups, in expanded notation, of units, tens, hundreds, thousands, ten of thousands, hundred of thousands, millions, etc..

Convert each of these subgroups separately to numbers written down with Roman numerals; Wrap it up: catenate these subgroups using the additive notation to get the end Roman numeral.
Example: 19 = 10 + 9 = 10 + (10 - 1) = X + IX = XIX; 39 = 30 + 9 = (10 + 10 + 10) + (10 - 1) = (X + X + X) + (X - I) = XXX + IX = XXXIX; 42 = 40 + 2 = (50 - 10) + (1 + 1) = (L - X) + (I + I) = XL + II = XLII; 79 = 70 + 9 = (50 + 10 + 10) + (10 - 1) = (L + X + X) + (X - I) = LXX + IX = LXXIX; 99 = 90 + 9 = (100 - 10) + (10 - 1) = (C - X) + (X - I) = XC + IX = XCIX; 104 = 100 + 0 + 4 = 100 + (5 - 1) = C + (V - I) = C + IV = CIV; 120 = 100 + 20 + 0 = 100 + (10 + 10) = C + (X + X) = C + XX = CXX; 150 = 100 + 50 + 0 = C + L = CL; 173 = 100 + 70 + 3 = 100 + (50 + 10 + 10) + (1 + 1 + 1) = C + (L + X + X) + (I + I + I) = C + LXX + III = CLXXIII; 184 = 100 + 80 + 4 = 100 + (50 + 10 + 10 + 10) + (5 - 1) = C + (L + X + X + X) + (V - I) = C + LXXX + IV = CLXXXIV; 200 = 200 + 0 + 0 = (100 + 100) = (C + C) = CC; 207 = 200 + 0 + 7 = (100 + 100) + (5 + 1 + 1) = (C + C) + (V + I + I) = CC + VII = CCVII; 267 = 200 + 60 + 7 = (100 + 100) + (50 + 10) + (5 + 1 + 1) = (C + C) + (L + X) + (V + I + I) = CCLXVII 448 = 400 + 40 + 8 = (500 - 100) + (50 - 10) + (5 + 1 + 1 + 1) = (D - C) + (L - X) + (V + I + I + I) = CD + XL + VIII = CDXLVIII; 503 = 500 + 0 + 3 = 500 + (1 + 1 + 1) = D + (I + I + I) = D + III = DIII; 944 = 900 + 40 + 4 = (1,000 - 100) + (50 - 10) + (5 - 1) = (M - C) + (L - X) + (V - I) = CM + XL + IV = CMXLIV; 1,973 = 1,000 + 900 + 70 + 3 = 1,000 + (1,000 - 100) + (50 + 10 + 10) + (1 + 1 + 1) = M + (M - C) + (L + X + X) + (I + I + I) = M + CM + LXX + III = MCMLXXIII; 2,019 = 2,000 + 10 + 9 = (1,000 + 1,000) + 10 + (10 - 1) = (M + M) + X + (X - I) = MM + X + IX = MMXIX; 3,999 = 3,000 + 900 + 90 + 9 = (1,000 + 1,000 + 1,000) + (1,000 - 100) + (100 - 10) + (10 - 1) = (M + M + M) + (M - C) + (C - X) + (X - I) = MMM + CM + XC + IX = MMMCMXCIX; 4,000 = 5,000 - 1,000 = (V) - M = M(V). The reading rules of the Roman numerals, summary: How to convert the Hindu-Arabic numbers to Roman numerals: breaking down the numbers into place value subgroups
Two lists of the first Roman numerals (in ascending order): Mathematical operations with Roman numerals: