In some countries this is illegal - the worst you can say about a person is "no comment."
In some countries this is illegal - the worst you can say about a person is "no comment."
edit: I always thought this is just culturally. I have no idea, but I don't believe there are any laws that forbid telling the truth about former employees.
Good: "Mr. X's technical competence is excellent. He solved difficult technical challenges quickly and correctly. His performance at our company exceeded our expectations."
Bad: "Mr. X handled assigned tasks adequately."
A long reference almost always means it is good. If it's longer than two pages, it's almost certainly excellent.
In Germany, employees can request a "simple" reference, which basically just certifies employment and does not contain any judgement. What I described above is a "qualified" reference; usually it is seen as a bad sign to produce just a "simple" reference and not a "qualified" reference.
One way to say something negative is to omit certain words, as you said. If your praise is less than exuberant, it is also a warning sign. Some years ago, employers used to mark that someone is in a union by ending their signature with a hook to the left. If you wrote someone is sociable, that was code for being alcoholic.
The funny thing is, the law states explicitly that the letter of reference must be unambiguous, and must not contain any hidden meaning.
I think this whole system is ridiculous and ripe for reform, but I don't think this will happen.
We're talking about an ex-employee here. It's not like giving negative feedback will really help the company. It can only hurt them.
Fortunately, lack of good information strongly implies negative information that is being withheld, so it's all fine.
In my case, my manager was particularly fearful about losing me. When I told him that I was entertaining other gigs he plainly told me that "I will black mark you across the industry if you leave." I was young, naive and scared, so I took it seriously (luckily things have improved vastly since). I now know that I would not put him on my CV; instead the myriads of other employees who have interacted with me. This is why those laws are in-place: preventing hostage situations.