One way of forming the aorist of a verb root involves reduplication using a consonant followed by a long or short vowel, usually preceded by a temporal augment a- (Whitney 1889:309-310). For the vast majority of verbs, the reduplicated aorist stem ends in a short vowel plus a single consonant. Since these aorists almost always add an a suffix (vas"a-, syada-), this makes the stem light, by either the CS or the OM theory. For these light stems (but not for the rarer heavy stems), the vowel of the prefix is short when the stem begins with two consonants (ádidyutat `shone', RV 6.11.4), and long when it is followed by only one consonant (anînas"at `drove away', RV 10.162.2). This makes sense if the actual requirement is for the prefix to be heavy, in which case a basic short vowel is lengthened only if the syllable is not already heavy because it is closed by the first of two consonants. The CS theory is validated in that any two consonants will do: all versions of the OM theory, including that of the Taittirîyapraâtis"âkhya, would syllabify á-di-dyutat, which would undesirably leave the prefix light (Vaux 1992:295).