The DualDisc is a type of double-sided optical disc product developed by a group of record companies including MJJ Productions Inc., EMI Music, Universal Music Group, Sony BMG Music Entertainment, Warner Music Group, and 5.1 Entertainment Group and later under the aegis of the Recording Industry Association of America (RIAA). It featured an audio layer intended to be compatible with CD players (but too thin to meet Red Book CD Specifications) on one side and a standard DVD layer on the other. In this respect it was similar to, but distinct from, the DVDplus developed in Europe by Dieter Dierks and covered by European patents. DualDiscs first appeared in the United States in March 2004 as part of a marketing test conducted by the same five record companies who developed the product. The test involved 13 titles being released to a limited number of retailers in the Boston, Massachusetts, and Seattle, Washington, markets. The test marketing was seen as a success after 82% of respondents to a survey (which was included with the test titles) said that DualDiscs met or exceeded their expectations. In addition, 90% of respondents said that they would recommend DualDisc to a friend. However, sales plummeted over the next three years, particularly in competition with rival formats like SACD and DVD-A discs. DualDisc titles received a mass rollout to retailers throughout the United States in February 2005, though some titles were available as early as November 2004. The recording industry had nearly 200 DualDisc titles available by the end of 2005 and over 2,000,000 units had been sold by the middle of that year. Technical details DualDiscs were based on double-sided DVD technology such as DVD-10, DVD-14 and DVD-18 except that DualDisc technology replaced one of the DVD sides with a CD. The discs were made by fusing together a standard 0.6 mm-thick DVD layer (4.7-gigabyte storage capacity) to a 0.9 mm-thick CD layer (60-minute or 525-megabyte storage capacity), resulting in a 1.5 mm-thick double-sided hybrid disc which contained CD content on one side and DVD content on the other. The challenge for the designers of DualDisc was to produce a dual-sided disc which was not too thick to play reliably in slot-loading drives, while the CD side was not too thin to be tracked easily by the laser. DVDplus, though conceptually similar, used a thicker CD layer and thus is more likely to get stuck