Another problem that kept the Plus/4 from selling was that even though the three machines (116, C16 and Plus/4) were all compatible with one another, developers tended to write programs for the lowest common denominator in a computer family. So as not to alienate buyers of the C116 and C16, which were intended to be the largest selling machines in this series, most software was designed to run in and the extra memory on the Plus/4 was not as widely supported as it could have been. Also, most development for these machines was in the less-lucrative European markets. Major software developers in North America continued to focus instead on the huge C64 market. Plus/4 software development in North America was mostly the domain of obscure companies who sold products via mail order, similar to other computers like the TRS-80 CoCo that did not have support from major developers. Since the Plus/4 accounted for most North American sales, NTSC software was usually designed to support 64K and disk drives unlike in Europe where software was most often distributed on cassette and designed to support a 16K machine.