There is a problem with the way variation among the types of strong noun is handled in current textbooks. Faced with the challenge of teaching a complex system of grammar to a generation of students who cannot afford the time to learn all its intricacies, textbook authors have generally opted to simplify the theory. Surely there is no avoiding this development, and it should cause no difficulty, were it not that some scholarly resources, such as the Dictionary of Old English, continue to rely on a fuller knowledge of grammatical categories. This is all as it should be, of course, but DOE does not in the Part of Speech section of its manual explain the traditional system, even though generations of students have now been trained without ever having been introduced to it. A brief explanation follows.
Mitchell and Robinson, in what is perhaps the most popular textbook of Old English today, introduce a variety of strong nouns distributed among five headings: "Strong Nouns like stān (masc.) and scip (neut.)"; "Masculine and Neuter Nouns in -e"; "Strong Feminine Nouns"; "Nouns Affected by i-Mutation"; and "u-Nouns" (§§33–62). The nouns in -e, as the authors point out, sometimes belong to the same declension as stān and scip, but some nouns with this ending belong to "another declension" that is not further explained by Mitchell and Robinson (and not all of whose members end in -e). This heading thus does not qualify as a declension of its own, but a subset of it does represent a distinct declension. In addition to these five important classes, Mitchell and Robinson list two further groups under inconspicuous section numbers §59 and §60, representing frēond and fēond (and many similar words that go unmentioned) and relationship-nouns, respectively, coming to a total of seven declensions. The noun types given in Baker's Introduction to Old English add up to a similar total, but among its number are also "Nouns with -r- plurals" (such as cild) and "Nouns with -þ- endings", which are an eighth and ninth class not or barely mentioned in Mitchell and Robinson; the ninth, in fact, was in the process of being absorbed by the other classes during the time of written Old English. Although not all divisions between these two textbooks fully correspond to each other, they mostly represent the same categories clothed in what each author considers the least offensive presentation.
The system that is behind these user-friendly categories is one based on historical differences, not all of which were equally visible in Old English by the time it was written down. The difference between stān and sunu is evident in the endings of the various cases and numbers, as is that between both these masculines and the feminine giefu; accordingly, these represent three different declensions. But declensions are not all about endings: stems are at least as important, and hence the category mentioned by Mitchell and Robinson as "another declension" in precise usage has the full status of a declension, even though its members generally behave just like stān in terms of their inflectional endings. These four classes are traditionally considered the major strong declensions, while the remaining five are with various justification referred to as "minor". Also as a consequence of differences in stem, the four major strong noun classes can be broken up into a finer subdivision of eight or so stem types, which in turn are distributed across the three genders in various ways. However, DOE does not expect its users to know this, and it does not distinguish between major and minor declensions either, simply numbering all classes consecutively in the order in which they appear in Campbell's Old English Grammar, the standard reference grammar of Old English (see my textbook writeup). Correspondences are as follows:
|Mitchell & Robinson
|strong nouns like stān (masc.) and scip (neut.)
|strong feminine nouns
|[merely gestured towards as "another declension" some of whose members end in -e]
|consonant stems, athematic nouns
|nouns affected by i-mutation
|nouns of relationship
|nouns ending in -r which denote relationship
|nouns in -nd-
|the nouns frēond and fēond
|stems in Indo-European -es, -os
A table of the major strong noun classes with their endings as well as their subdivision into stem-types may be found here (given funding, I hope one day to make the table fully interactive). Weak nouns are normally considered a single declension, displaying variation only between the genders.