'despicable'. The Poet Laureate holds that in these words Milton kept
the long Italian _a_ of the penultimate or secondary stress.

Fall'n Cherube, to be weak is miserable.

In English we have naturalized _-able_ as a suffix and added it to
almost any verb, as 'laughable', 'indescribable', 'desirable'. The
last word may have been taken from French. The form 'des[)i]derable'
occurs from the fourteenth to the seventeenth century. Originally
'acceptable' threw the stress back, as in Milton's

So fit, so acceptable, so Divine,

but the double mute has brought it into line with 'delectable'.
Nowadays one sometimes hears 'dispútable', 'despícable', but these
are intolerable vulgarisms.

SUFFIXES IN T[)I]LI AND S[)I]LI. These words mostly lengthen the _i_
and make the usual shortenings, as 'missile', 'sessile', 'textile',
'volatile', but of course 'futile'. Exceptions which I cannot explain
are 'foss[)i]l' and 'fus[)i]le'.

SUFFIX IN [=A]LI. These adjectives shorten the _-a_ and, with the
usual exceptions, the preceding vowels, as 'dóctrinal', 'fílial',
'líberal', 'márital', 'medícinal', but of course by the 'alias'
rule 'arb[=o]real' (not a classical word in Latin) and 'g[=e]nial'.
Words like 'national' and 'rational' were treated like trisyllables,
which they now are. The stress is on the antepenultima except when
heavy consonants bring it on to the penultima, as in 'sepulcral',
'parental', 'triumphal'.