These three forms may look similar, but they are not equally suitable for writing about a decade. The main questions are whether your reader knows the century and whether the spelling is standard.
- 1990s names the years from 1990 to 1999. It makes the century clear, so it is a good choice in history writing or whenever several centuries are discussed. For example: The company expanded during the 1990s.
- 90s is a shorter, informal form. In a conversation about recent music or fashion, readers will usually understand it as the 1990s. However, the century depends on context. For example: She collects 90s pop albums. The form ’90s uses an apostrophe to show that the opening digits have been left out.
- 90ies is not a standard way to write a decade in English. With digits, add s, not ies. If you want words instead, write nineties or nineteen nineties. For example: He moved abroad in the nineteen nineties.
Apostrophe position also matters. In ’90s, the apostrophe comes before the number because it marks omitted digits. A decade name does not need an apostrophe before its final s: write 1990s, not 1990’s, when simply naming the decade.
For an international audience or a text comparing historical periods, the full four-digit form avoids making readers guess which century you mean.