Did you know that a period is never
used with metric units, (e.g., km, ml, kg)
or in acronyms (e.g., laser, NATO, scuba)?

When monks began to copy out religious texts In the 6th century CE, they used all capital letters, with no spaces or punctuation. This slowed the copying, and made reading the material aloud difficult. Although the monks developed a cursive style to speed up the copying, reading was still a challenge – so they added a dot between each word. However, that made it difficult to discern where a thought ended. So, they replaced the dots between the words with spaces.

Eventually, the period was used to indicate the end of a complete thought.