Like the contractions previously studied, these contractions are not used where part of the letters of the contraction fall into a prefix and the rest into a root or base word. Thus, the sign for ed is not used in reduce, deduce, edict, predict, or predate. Similarly, the sign for er is not used in derogatory, prerogative, erect, erupt, rerun, or derail. The sign for ble is not used in sublet, nor the sign for ar in infrared.
In a few words, the beginning letters se constitute a prefix meaning "apart." When that prefix is followed by the letter d, as in seduce and seditious, the ed contraction cannot be used because it would overlap between the prefix and the root word.
An exception to this rule occurs in a few familiar words that begin with the letters ar. In the easily read words around, arise, and arose the ar contraction is used, even though in these cases the a is a prefix. [X.34.c]
• Note: Remember the very strong rule that says that when all of the letters of a contraction fall into the same syllable, the contraction is used. Although the er contraction cannot be used in derive because de is a prefix, it is used in derivation because both the e and the r fall into the same syllable.