Stems

Every noun (and adjective) is assigned to one of a number of declensions, each with its own pattern of inflections--though there are a lot of similarities between declensions. Which declension a word belongs to depends on its etymology. The names of the declensions (a-stem, o-stem, in-stem, etc.) refer to the hypothetical form of the word in Proto-Germanic, the ancestor of Gothic, English, German, Norse, etc. The stem was attached after the root of a word, and after the stem came endings indicating case / number.

Thus the PG noun *dagaz "day" consisted of a root DAG- plus the stem -A- plus the nominative singular ending -Z. But PG *gastiz "guest" had the stem -I-. In Gothic these stem vowels were lost or reduced in the some declensions, hence Go. dags, gasts, but still present in the accusative plurals, Go. dagans, gastins. A stem vowel may also appear when a word is used as the first part of a compound, e.g. áina-baúr "only born", but in other words is omitted: áin-falþs "simple".

"Consonant stems" include words with a stem that originally ended in a consonant (an-, on-, in-stems), as well as a smaller number of words that added the inflectional endings directly to the root.