Outside the computer industry 'compatibility' is usually something that troubles lovesick damsels. Will their suitors be suitable? In the US, cuckoo's nest of the computer industry, they have a typically pragmatic answer - blood tests.

Where computers are concerned, compatibility means a system behaves like another. Not any old other - but usually one with greater marketing potential, or more ancillary equipment. The target is usually IBM. Variety may be the spice of life but the prospect of lonely bankruptcy forces manufacturers into conformity.

As with romance, the course of true micro compatibility doesn't always run smoothly. Thanks to another American habit - a cavalier imprecision over the meaning of words - compatibility can mean many things. Here be dragons... or more to the point, here be no adequate substitutes for blood tests.

Consequently, anybody who opts for IBM standards has to deal with degrees of compatibility - unless they go for the real thing.

A number of features are wheeled out as evidence of compatibility. Machine A has an Intel 8088, Machine B runs MSDOS 2.0, Machine C has been seen running the Microsoft Flight Simulator - an essential business tool - while Machines D, E and F have IBM-compatible price tags.

Some claims of compatibility are brazen attempts to pull the wool over consumers' eyes, but as the art has advanced and commonly attained levels of compatibility have risen the more blatant deceptions are now rare.

And if it's any consolation, if you've bought an MSDOS system on the understanding that it's PC-compatible you aren't the first to be taken in by the latitude the term permits. In the mainframe arena IBM imitation is an industry so well developed that its participants have their own abbreviation - the PCMs (Plug-Compatible Manufacturers).

For a micro user the problem is pronounced. Seven times out of ten it's not a corporate budget meeting the cost of the system, and the price of failure (of a computer system) is at least as great in a small business. How can you avoid buying a 'compatible' that isn't compatible?

There are different ways of defining compatibility with the IBM PC. In the US IBM clones are commonly categorised as operationally compatible, data compatible or MSDOS compatible. Software is said to exhibit degrees of mmisbehaviour, ranging from docile to downright unruly in the way it makes contact with the operating environment.

An operationally compatible machine runs almost any IBM software and accepts peripherals or expansion units designed for the PC. A data compatible machine can use IBM disks but rejects some of the software, like a body that won't accept a transplanted heart. An MSDOS compatible system will run software written for the MSDOS environment but with disk formats peculiar to the system.

IBM has been unusually open in the way the PC has been constructed - certain features remain proprietary but, by and large, it has encouraged independent suppliers to produce PC equipment. On its future plans IBM is less than forthcoming.

A particularly popular belief has it that MSDOS compatibility will soon become about as valuable as narrow-gauge rolling-stock on an Inter-City line. The speculation, idle as it may be, indicates that IBM's PCDOS will gradually move away from its roots in MSDOS, leaving this level of compatibility high and dry.

This is the horse Digital Research is backing. The producer of CP/M was left out in the cold when IBM chose Microsoft's MSDOS to grace the PC, but now it intends to stage a comeback with the PCDOS-compatible element of Concurrent CP/M 3.1. So far this relates to PCDOS 1.1; it handles the file format, and some system-dependent functions of PCDOS are implemented, so applications that bypass the basic disk operating system will also run. It also leads to multi-user applications, no small consideration for future expansion.

With this in mind (the possibility that today's hand-in-glove compatibility might be chalk and cheese tomorrow) means rating the degree to which various systems live up to their makers' claims would have limited uses. But it may become possible through Standard Telephones and Cable (STC), which launched its Xtra with an attention-grabbing scheme to test the degree of its competitors' compatibility.

"The buyer doesn't understand the difference between functional and operational compatibility and the rest," said STC's Stewart Goldberg, commenting on research by ITT in the US. "He understands compatibility as meaning that software will run."

"The challenge is to highlight what people should be looking for," he said.

STC will concentrate on software, looking at what will and won't run. "We'll publish a list of what won't run. "We'll publish a list of what won't run," said Mr. Goldberg. "It's likely to be in the games area, where people are writing down to the chips all the time." He said: "Where people go down to the ROM Basic you've got a problem - not if it calls Basic in certain ways, but if it goes below to something that is absolutely IBM-proprietary." It may be for this reason that such software is called 'naughty'. You can call it what you like; there are no standards in this area yet.

The question of software interaction is what compatibility boils down to. PCN covered this in the context of Compact Accounting Services' conversion program (Issue 5) - but the game has moved on and the number of players has grown enormously.

ITT's US survey put the IBM PC's share of the new software at 85 per cent of the total. Some of this vast output will become available for other machines, but a manufacturer with genuine operational compatibility has a head start. ITT has originally aimed lower than this with what became the Xtra, and Mr. Goldberg acknowledged that it wasn't enough: "On a monochrome display, we'd implemented a more efficient mapping technique. Certain programs, like Multiplan, would need porting (they go below the BIOS to the screen driver). We were also in difficulty with a diskette adaptor."

Eventually, the Xtra will live up to its name more fully by incorporating the kind of features that ITT, primarily a communications company, specialises in. "You don't remove compatibility by adding a telephone, and that's the kind of thing we'll be developing," said Mr. Goldberg. Other manufacturers are also going beyond IBM: Tandy's Model 2000 uses the Intel 80186 processor, a true 16-bit device that's faster than the IBM PC's 8088; Televideo's PC has twice the graphics resolution; Sperry's can display 256 colours.

Among the rest the most notable is perhaps Compaq's portable machine, which anticipated an IBM development and set a separate trend.

Additional features are usually combined with a 10 to 15 per cent price advantage to lure you from the security of IBM to the world of the PCMs. In the mainframe market there are many users of Amdahl, National Advanced and other systems who are perfectly happy with their machines and will probably continue so for years.

There's no reason why the pattern shouldn't be repeated with micros - but let the buyer beware that he and the salesman mean the same thing when using the word 'compatibility'.