By arranging the vowels according to these four stems we arrive at the following system:—
| i. | ii. | iii. | iv. | |
| I. | ī | ei, ē | i | i |
| II. | ie | ou, ō | u | o |
| III. | i, ë | a | u | u, o |
| IV. | ë | a | ā | o |
| V. | ë | a | ā | ë |
| VI. | a | uo | uo | a |
[Note.—1.] On the difference between ei and ē, see [§ 17]; ou and ō, § 18; and in Series III i and ë, § 14; u and o, § 15.
[2.] Strong verbs belonging to Series II have iu in the indicative pres. singular; and strong verbs belonging to Series III-V with ë in the infinitive have i in the indicative pres. singular (§§ [14], [16]).
Examples.
| I. | snīden, to cut | snīde | sneit | sniten | gesniten; |
| līhen, to lend | līhe | lēch | lihen | gelihen. | |
| II. | biegen, to bend | biuge | bouc | bugen | gebogen; |
| sieden, to seethe | siude | sōt | suten | gesoten. | |
| III. | binden, to bind | binde | bant | bunden | gebunden; |
| hëlfen, to help | hilfe | half | hulfen | geholfen. | |
| IV. | nëmen, to take | nime | nam | nāmen | genomen. |
| V. | gëben, to give | gibe | gap | gāben | gegëben. |
| VI. | graben, to dig | grabe | gruop | gruoben | gegraben. |
For further examples see the various classes of strong verbs §§ 76-86. Class VII of strong verbs embracing the old reduplicated verbs ([§ 87]) has been omitted from the ablaut-series, because the exact relation in which the vowel of the present stands to that of the preterite has not yet been satisfactorily explained.
Other Vowel Changes.
[§ 13.] Most of the following vowel changes took place in prehistoric times; but as they play an important part in the verbs and word-formation, &c., we shall give them here.
[§ 14.] ë (= Indo-Germanic e) became i in the prehistoric period of all the Germanic languages:—