Blue Book 5A5
Para 2 states: "It is always permitted to vary certain parts of a system according to position and/or vulnerability. This includes, for example, variable NT openings and playing four or five card majors in different positions."
Are there any guidelines as to how much variation is allowed?
A pair plays 1NT 12-14 Vul, 11-14 may contain a singleton NV.
The range differential is OK, but the singleton? It seems OK to me but how do I justify that?
Comments
You could only enforce a ruling of different systems at the cost of proper disclosure - they would just says that Vul 1NT may contain a singleton and then 'never' have one.
In practice, what the regulation would not permit is changing between non-forcing 1C/1D and one of 1C/1D being (artifiial and) forcing.