When referring to a decade, such as the 1920s, the same criteria applies. It is quite acceptable to place a letter after a number without using an apostrophe. If, on the other hand, you remove the 19 from the 1920s, you would substitute an apostrophe to indicate that something is missing: the '20s. The 1930s could be written as well with out an apostrophe, but it would not make sense because there are not even three digits in this example. The 1940s could also be written without an apostrophe because there are at least four digits involved. The 1950s could be written with out an apostrophe because there are at least five digits. And so on. In general, if there are more than two digits before the year zero (00's, 10's, 20's, 30's, etc.) then you should put an apostrophe after the year. If there are only two digits before the year (02's, 07's) then you can omit the apostrophe. Here are some examples of how you can write the Roaring 20s: 1922, 1927, 1936, 1945, 1955. There are also books available that contain articles about various periods in history, for example, "The Roaring Twenties" by Charles Gehring or "A History of the Modern World" by William H. McNeill.