there is usually an inner protective cover to protect against dust and handling. This is normally shaped to allow it to readily slide within the outer cover. The inner sleeve is either thin white paper, either plain or printed with information on other recordings available from the same company, or a paper sleeve supporting a thin plastic bag. These quite often have a circular cut out so that the record label can be read without directly handling the record, though when the inner sleeve is printed with lyrics, which became quite common, then there is usually no hole. Decca Records used a system of colour-coding on these sleeves where a blue color denoted a stereophonic recording while red denoted a monophonic recording (the mono record players of the time were not always compatible with stereo records). This system was begun in the 1960s to reduce packaging costs. Packaging formats for compact discs widened the variety of presentations as well, even as the size of the CD meant that album covers were no longer so large. Besides the practicalities of identifying specific records, album covers serve the purpose of advertising the musical contents on the LP, through the use of graphic design, photography, and/or illustration. An album cover normally has the artist's name, sometimes in logo form; and the album title. Occasionally, though more common on historical vinyl records, the cover may include a reference number; a branding (the label), and possibly a track listing. Other information is seldom included on the cover, and is usually contained on the rear or interior of the packaging, such as a track listing together with a more detailed list of those involved in making the record, band members, guest performers, engineers and producer. On the spine of the package, the artist, title, and reference number are usually repeated so that albums can be identified while tightly packed on a shelf. Parental advisory labels are warning labels that are required to be placed on album covers when the music on the album contains explicit content such as vulgar language. These labels have been known to be controversial when it comes to keeping underage audiences from this content. There are a few different theories on this, such as the "forbidden fruit" and "tainted fruit" theories. The "forbidden fruit" theory states that when a child sees the parental advisory label on an album cover they will be