Ximenes was a cryptic setter from way back, and one of the many things he did was to set up the standards of fair play in cryptic crosswords. In 1966 he wrote the seminal book Ximenes on the Art of the Crossword (which is still in print).
Basically, while a cryptic clue can deceive, of course, in the end it has to actually say what it means (in terms of the wordplay) and be fair to the solver. Clues that follow his standards are said to be Ximenean (and this is what I do my best to write). The Times cryptic is the bastion of a pure Ximenean cryptic crossword.
There is a new camp, though, where these rules are seen more as guidelines, boundaries are stretched, and rules are broken ... these are said to be non-Ximenean or Libertarian clues. Many cryptic crosswords tend to have a smattering of Libertarian clues in them, some more than others, so it's good to have some idea of what Libertarian devices look like.
For starters, the most commonly noticed thing is the way they indicate letters and abbreviations, and some forms of wordplay. Some are certainly clever and cute, and other are less fair, in my opinion. Here are just a few examples of Libertarian 'abbreviations' :
- midday = A (the middle letter of day)
- Gateshead = G (the head of Gates)
- firstborn = B (the first letter of born)
- infer = put something in the letters fer
- finally = Y (final-ly, so the final letter of ly)
- not = no T (delete T from another word)
- exploits = PISTOL (an anagram indicated by ex- of the fodder -ploits)
I don't mind the first three of these examples, but do feel that infer, finally, not, and exploits are pushing the limits of fairness to the solver.
Libertarian clues can also be more wordy than Ximenean clues, with more linking words and some 'padding' (words which don't have any wordplay 'use') to make the clue's surface meaning read better. This sort of padding is frowned upon in Ximenean circles.
Some abbreviations are deemed to be Libertarian, such as note = A, B, C, D, E, F or G (as in the notes of the musical scale) ... while note = DO, DOH, RE, MI, FA, SO, SOH, LA, LAH, TI is Ximenean. I don't mind either, really.
However, the word many in a Libertarian clue can abbreviate any combination of Roman numerals that adds up to a big number, so CL (150), MMI (2,001), CM (900), MD (1,500), and a large number of other possibilities (which I do think is unfair to the solver, as there are too many possibilities).
Proper names may not have a capital letter, which may cause extra confusion when a famous person is being referred to. A clue may read as "put Word A into Word B", but in fact you have to put Word B into Word A. And so on ... so, in general, the rules are rather stretched and dodged at times.
For me, it comes down to ensuring that cryptic clues are enjoyable, a fun challenge to solve, but are also fair to the solver, so they can be solved. I do very occasionally use Libertarian devices in my clues — I suspect there is a spectrum across setters — but I'm always careful to make sure that my clues are fair, and mostly write Ximenean clues.