How did Old Saxon show features of both North Sea Germanic and South Germanic?

The Old Saxon language is considered a transitional dialect within the West Germanic group, reflecting a mixture of features that align it with both North Sea Germanic and South Germanic.1

Its dual nature is evidenced by the following characteristics:

• North Sea Germanic Affinities: Old Saxon shares specific phonological developments with North Sea Germanic languages like Old English and Old Frisian. For example, it preserves the voiceless stops (p, t, k) that were otherwise shifted to affricates or fricatives in Old High German. Additionally, it participated in the loss of final z in monosyllabic words, a trait characteristic of the North Sea group.21
• South Germanic Affinities: Old Saxon also exhibits features shared with South Germanic (Old High German), such as the replacement of inherited second-person singular past tense forms of strong verbs and the consistent change of ð to d in all positions.1

Because it occupied a geographic and linguistic position between these two groups, Old Saxon serves as a primary example of how Germanic dialects transitioned gradually rather than splitting into abrupt, isolated branches.1