Capitalization rules tend to vary by language and can be quite complicated. In English, for example, the first word of a sentence and all proper nouns are always capitalized. However, what is not so clear is the origin of the upper case distinction that has become common practice, especially in regards to Modern English.
To unmask the origin of the capital letter we need to go back to beginning of the 3rd century AD when Latin and Greek scribes were using a script called Uncial (Uncial is a majuscule script … majuscule is a synonym meaning “large or capital letter”). So basically everything was written in capitals.
The original twenty-one letters in the Latin alphabet are derived from the uncial style of writing. As the Latin alphabet was adapted for other languages over time, more letters were added that also incorporated the majuscule lettering thus giving us the Modern Latin alphabet from which the English alphabet is derived.
Medieval scribes spent all day, every day transcribing text. As they were going along, writing perfectly and quickly, the shape of the letters slowly changed to be more efficient.
Eventually, around the 9th century AD, as the uncial script evolved, a smaller, more rounded and connected Greek-style lettering called minuscule was introduced. This transformation gave us both capital and lowercase letters.
It soon became very common to mix miniscule and some uncial or capital letters within a word, the latter used to add emphasis. In contrast, many other writing systems such as Arabic make no distinction between upper and lowercase lettering – a system called unicase.