The RHS is the International Cultivar Registration Authority for lilies, and publishes a register of cultivar names used in the genus.
The Seventeenth Supplement to the International Lily Register (published in 1999) introduced a system of dual subdivisions for every newly registered lily. Prior to this publication, subdivisions had only been used in some of the horticultural divisions into which the genus has been split and the characters involved varied with the division created.
Now, both flower habit ((a) up-, (b) out- or (c) down-facing) and flower form ((a) trumpet, (b) bowl, (c) flat or (d) recurved) are indicated for each entry when this information has been supplied by the registrant. The existing lettering is retained with flower habit given first, separated from flower form by a forward slash mark (/). Thus the entry for 1(b/c) 'Aloft' represents an Asiatic Lily (Division 1) with out-facing, flat-faced flowers.
At present, there is no intention to systematically revise all existing Register entries to include a dual subdivision but when the relevant information is available, it will be added to the record.
It is not a requirement that dual submission be used at all times. The way in which the information is presented means that, where for a particular purpose it is necessary, one or both can be ignored. Their inclusion, however, together with the division number, helps to present, in an abbreviated form, a quick 'picture' of the flower concerned. For a full account of any plant, it is necessary to refer to the description.
Download registration forms here:
Application form for registration of a lily name (pdf)
Application form for registration of a lily name (doc)
The RHS publishes The International Lily Register and Checklist 2007 and subsequent supplements, some of which are available as downloads from the link below.