Masculine a-stems are almost all inflected the same, as in "hund" ("dog") below. The neuter a-stems, however, are split in two: some of them end in "-u" in the nominative/accusative plural, while others have no ending there at all. This was caused by a sound change called high vowel apocope, which occurred in the prehistory of Old English. Short "-i" and "-u" disappeared at the ends of words after a heavy syllable—that is, a syllable containing a long vowel or long diphthong or ending in two or more consonants—and after two light syllables. Nouns which kept short -i/-u are called light, while nouns which lost them are called heavy.