Example:
A B C D E F G H I K L M N O P Q R S T V W X Y Z
In the Roman alphabet, the I equals the J and the V equals the U. Also, they only used capital letters.

Our alphabet is almost similar to the Roman alphabet. It is derived from the Greek alphabet. The Roman people didn't use any interpunction (comma's, periods...) or spaces. They just wrote each letter after the other. Also, they only used capital letters. Lowercase letters weren't used until after the 8th century. You can understand that it must be quite hard to read a text in Latin.
The Romans didn't use numbers, too. They used a letter to symbolize a number.

• I = one
• V = five
• X = ten
• L = fifty
• C = hundred
• D = fivehundred
• M = one thousand

For example:
XI = 11 (X + I = 10 + 1 = 11)
CXVIII = 118 (C + X + V + 3xI = 100 + 10 + 5 + 3x1 = 118)

If a letter symbolizing a value is placed before a greater value, the smaller value is substracted from the big value.

For example:
IX = 9 (X-I = 10-1 = 9)
XXCIII = 83 (C-2xX + 3xI = 100-2x10 + 3x1 = 83) MCMXCIX = 1999 (M + M-C + C-X + X-I = 1000 + 1000-100 + 100-10 + 10-1 = 1999)