Disyllabic adjectives ending in
-y can take either the analytic or synthetic comparative. Offhand, I can't think of any exceptions.
This is also true of a number of other disyllabic adjectives including
simple,
clever,
common,
quiet,
narrow, and
stupid. Others with a similar phonetic shape take only the analytic (at least I've never heard ?
certainer or ?
vapider in my life even though Wiktionary has entries for them).
Most monosyllabic adjectives allow only the synthetic form, but even here there are exceptions, e.g.
more loath,
more worth,
more chic, . (Again, Wiktionary lists ?
chicer but I've never encountred this IRL.)
For a fairly comprehensive analysis:
http://www.diva-portal.org/smash/get/diva2:6690/FULLTEXT01.pdf.
"Richmond is a real scholar; Owen just learns languages because he can't bear not to know what other people are saying."--Margaret Lattimore on her two sons