I think there was a question on this very recently but I don;t think it got an answer and I can't find it to reply to, so apologies for the extra noise. I have a large text with floats numbered sequentially by chapter and then figure/table number, but at the last minute I've had a request for two figures in a particular chapter to become 27.1a and 27.1b instead of 27.1 and 27.2. It's not a combination - they are separate figures on different pages.
I've experimented with the options I can see, but it's not obvious if there is a facility to override the numbering of two figures in a stream of normally numbered figures like this. I was thinking of the equivalent of the old \sym{symbol} command that used to work as a way to override a single item in an otherwise sequential list.
I guess there's a brute force method of setting up a new type of figure with the label "Figure 27.1" and having them numbered as lowercase alpha, but if there's an easier way that would be great.
Thanks.