An audio cassette is a type of cassette which can store music and sounds. To play a tape, a cassette player or cassette recorder is used. This is also known as a cassette deck, by analogy with reel-to-reel decks. Cassettes store the sound on a magnetic tape that is wound around the two reels in the cassette. Like vinyl records, most cassettes have two sides, called the A-side and B-side. The listener flips the cassette over to play or record on the other side. Some players have auto-reverse, meaning they can play both sides on their own. Cassettes have a tab in the upper left corner that can be removed to prevent accidentally recording over the tape. Cassettes typically hold anywhere from a few minutes up to an hour of audio per side. The longer the tape, the thinner it must be. Tapes longer than 60 minutes (30 minutes per side), and especially those longer than 90 minutes (45 minutes per side), tend to be fragile and more likely to break or get jammed inside the player.

History 
The standard audio cassette was invented in 1962 by Lou Ottens of the Philips company. They named it the "Compact Cassette". The first cassettes and cassette recorders had poor audio quality. They were used for recording speech rather than music. Over time, audio quality improved, and they became usable for music. During the 1970s, the cassette grew to be a popular physical audio format. Their popularity grew further during the following decades. The Walkman, released in 1979, made the cassette very popular in the 1980s. In the 1990s, CDs became more popular than cassettes for prerecorded music, however the cassette remained the most popular format for homemade recordings until CD recorders became affordable in the early 2000s.

Types of cassette 
There are four types of cassettes, each using a different magnetic particle bonded to the plastic tape stock. The tape type alone is not an indicator of the quality of the tape.
 Type I - Ferric - Type I was the original cassette formulation and is made from rust-like ferric oxide particles. These particles give the tape its characteristic brown color. Type I tapes have strong, high-quality bass and midrange, but less treble response than the other types.
 Type II - CrO2 and cobalt - Type II tapes are made from chromium dioxide or a mixture of cobalt and ferric oxide particles. The tape is typically black or very dark brown. Type II tapes have good treble response, and acceptable bass response, although the bass is not as strong as Type I.
 Type III - Ferrochrome - Type III tapes are a hybrid of Types I and II. They consist of a layer of ferric tape for strong bass, and a layer of chromium dioxide tape for strong treble. Type III tapes are no longer made and were never very popular. Many tape recorders do not have a setting for them. On decks without a Type III setting, they should be recorded as a Type I.
 Type IV - Metal - Type IV tapes are made from metal particles rather than metal oxides. The tape is typically a shiny metallic black color. They have good treble and bass response and very high dynamic range. Type IV tapes are no longer made.

Decline in popularity 
As compact discs grew in popularity, cassettes were used less. In 2011, the Oxford English Dictionary attempted to remove the word "cassette tape" from a small version of its book. Cassettes are still popular with some audiophiles, as it is an analog format that is portable and more convenient than vinyl.

References 

Magnetic tape