Nostradamus didn't start out with the intention to write a book of poetry. Instead, he spent his nights in the attic in his home in Salon, France. This attic became the retreat where he studied his favorite topics, read, and pursued his interest in astrology. It was also here that Nostradamus began using meditation techniques and a prayerful attitude to ask for visions of what the future might be. He wrote notes and even made sketches of the visions he saw. He then transformed the notes into the poetry form that exists today as his prophecies.
A quick lesson on quatrains
A quatrain is four lines of poetry that are grouped together. Sometimes these lines are set apart by spacing, and sometimes because they all rhyme. The quatrain your teacher taught you as part of an English lesson is the same structure Nostradamus used as a lesson on life for the rest of mankind in The Prophecies.
Each quatrain Nostradamus wrote is a separate piece and not part of one long poem; so don't sweat it if you look at things that seem to be out of order, because they're not (in order that is). The quatrains were meant to stand alone. Rhyme, while not a requirement, seems to have been one of Nostradamus's elements. Most of his quatrains contain rhymes between the first and third lines and the second and fourth lines. This kind of weaving together of rhyme makes poetic lines feel like they're built strongly to stay together.
Nostradamus probably wrote the quatrains in a rough form of Latin and then translated them into a mix of French, Provençal, Italian, Latin, and made-up words that suited his purposes. Each one of these four-line wonders spoke of at least one future event and sometimes several that could be related.
When a century isn't 100 years
Nostradamus arranged his prophetic quatrains into ten groups of 100 — well, almost. Century 7 has only about half its quatrains, and no one has figured out why Nostradamus shorted this century). These ten groups are called centuries, but don't get derailed into thinking that these grouped prophecies covered 100 years. Here, centuries have nothing to do with years and everything to do with keeping 942 prophecies organized somehow.
In terms of Nostradamus's prophecies, century simply refers to the 100 quatrains into which most of the prophecies were divided. Nostradamus didn't pull this format out of a hat. His knowledge of numerology and the power of numbers gave him a distinct form for his message.
The most common way to note which of the 942 quatrains is being referenced is to identify them by both the century and the quatrain number. Century numbers are typically in Roman numerals, and the quatrain number follows after a dash. For example, C II – 45 refers to Century II (2) and Quatrain 45.
An exception to every rule
Just when you think you have quatrains and centuries all sorted out, you'll find a wrench in the works. Nostradamus wasn't a great poet. The overall form of his poetry would've given his mentor and noted critic of poetry, César Scalinger, quite a fit. Frankly, the rhymes were rough.
Consider the idea that a poem normally has a beginning, middle, and an end. You'd expect a poem (if the collection of quatrains in The Prophecies are considered together to be one piece) that covers many years to at least have a regular sort of timeline, but Nostradamus didn't give readers that comfort. The quatrains appear in a seemingly random order that certainly isn't guided by time, and topics are spread throughout the entire collection in quatrains that are separated instead of being collected in nice, neat bundles where all the quatrains in one section talk about a specific time or a specific topic. But even the exception has an exception, and in several instances, a prophecy continues from one quatrain to another in a series.