A little more about Old Germanic poetry. (Part One here.)
While this overall poetic structure dominated the various Germanic languages, still the sundering of the tribes that resulted in different tongues also resulted in different poetic styles. This was done not so much by discarding the basic rules, as by working within them and at times adding additional ones. The main difference seems to have to do with the relative wordiness or terseness of the language. Old Saxon, for example, was much wordier than Old Norse or even Old English at times. Reflexive pronouns were more common, so that where OE would have ongann, ‘began’, OS would have bigan imu (thus doubling the syllable count). Moreover, even where OE and OS prose syntax might be similar, OS poets were more likely to fit more into a single half-line. Thus, while there are still only four main stresses per line, there were a great deal of unstressed syllables, making their verse long and rather belabored. The following line has seven weak syllables before the first strong one (note alliteration on the w-sound):
bigan imu an themu uuege uuahsen; thô it eft thes uuerodes farnam,
it began to grow on the path; then it afterwards [was] destroyed by the people, . . .
Old Norse was terse by contrast, expressing the same ideas in very few syllables. This led to a lot of stylizing by later poets, who revelled in the beautiful efficiency of their language and liked to explore just how succinct they could make it. To the existing rules of alliteration and meter they added further rules concerning syllables count and internal and external rhyme. Notice the almost complete lack of weak syllables in lines like these from the Hávamál (strong syllabes are bolded):
Deyr fé, deyja frændr
Die cattle, die kin. . . .
Ár skal rísa sá er annars vill
fé eða fjör hafa;
Early must he rise who would another’s
life or wealth have.
More in a subsequent post on the many and various ways poets devised of "using the Norse."
Old English might be thought of as midway between Old Saxon and Old Norse, both in terms of wordiness and vocabulary. (And historically, in a way, too: The Anglo-Saxons had their roots on the continent, yet had more Norse influence than their cousins back home.) Thus the OE flavor of poetry demanded at least one weak syllable for each strong one (thus four syllables minimum per half-line), and in fact usually had between 1 to 2 weak syllables for each strong one. Here is a sample of OE poetry from the poem-fragment of the Battle of Maldon:
"Gehyrst þu, sælida, hwæt þis folc segeð?
Hi willað eow to gafole garas syllan, . . .
Hear thou, seaman, what this people sayeth?
They will give you a tribute of spears, . . .
The great exception to this rule in OE poetry is the long verse. This is a phenomenon that contains three strong syllables per half-line, something that otherwise only occurs in Norse poetry. These long verses occur always in pairs (always with another long verse on the same line), and usually in groups. Here is a passage from the Dream of the Rood that has both long and regular verses (strong syllables are bolded):
"Þæt wæs geara iu, (ic þæt gyta geman),
þæt ic wæs aheawen holtes on ende,
astyred of stefne minum. Genaman me ðær strange feondas
geworhton him þær to wæfersyne, heton me heora wergas hebban.
Bæron me ðær beornas on eaxlum, oððæt hie me on beorg asetton,
It was years ago (I remember it still)
That I was hewn down at the edge of the forest,
cut from my trunk. Strong foes took hold of me there,
there made for themselves a spectacle of me, commanded me to carry their criminals.
There warriors bore me there on their shoulders, until they set me on a hill.
The Silent Tribes
The languages of many ancient Germanic tribes (Burgundians, Vandals, Gepids, etc.) are not fully known, at least not as distinct from other larger and closer tribes. Gothic, Old Frisian, and Old (Low) Franconian, while known languages with their own extant texts, nevertheless are not mentioned here because no poetry from these languages has survived. Still, one is probably safe in guessing that if they had, they would follow the same pattern. This guess led Tolkien to imagine what it would have sounded like, hence his composition of Bagme Bloma. (Here's another poem composed by a modern Gothic enthusiast, which in fact conforms more closely to Germanic alliteration rules than Tolkien's poem.)