The final major development of the typewriter was the electronic typewriter. Most of these replaced the typeball with a plastic or metal daisy wheel mechanism (a disk with the letters molded on the outside edge of the "petals"). The daisy wheel concept first emerged in printers developed by Diablo Systems in the 1970s. The first electronic daisywheel typewriter marketed in the world (in 1976) is the Olivetti Tes 501, and subsequently in 1978, the Olivetti ET101 (with function display) and Olivetti TES 401 (with text display and floppy disk for memory storage). This has allowed Olivetti to maintain the world record in the design of electronic typewriters, proposing increasingly advanced and performing models in the following years. In 1981, Xerox Corporation, who by then had bought Diablo Systems, introduced a line of electronic typewriters incorporating this technology (the Memorywriter product line). For a time, these products were quite successful as their daisy-wheel mechanism was much simpler and cheaper than either typebar or Selectric mechanisms, and their electronic memory and display allowed the user to easily see errors and correct them before they were actually printed. One problem with the plastic daisy wheel was that they were not always durable. To solve this problem, more durable metal daisy wheels were made available (but at a slightly higher price).