Refers to a hardware or software system that can successfully use interfaces and data from earlier versions of the system or with other systems. For example, a new version of sequencing software designed so it can properly read files from older versions is backward compatible. Nowadays this type of compatibility is taken for granted with software, but it wasn’t always so easy. The downside of too much backward compatibility is that software can tend to get bloated and inefficient by having to deal with too many prior formats. With hardware it is much more expensive to maintain a high degree of backward compatibility. Think how much a computer would cost if it had to have SCSI, ADB, serial ports, parallel ports, IDE, NuBus, ISA, PCI, USB, and FireWire compatibility. Backward compatibility is more easily accomplished if the previous versions have been designed to be forward compatible.