A number of sound changes occurred during the German Consonant Shift. Among these changes were the syllable initial consonant clusters "sp" and "st". The voiceless alveolar fricative became an alveopalatal voiceless fricative.
These changes did not occur in all varieties of German- they occurred in the variety of German known as High German or Hochdeutsch. In Low German, they did not occur. For example, the words "Strasse" (street) and "Sprache" (language) are pronounced with an alveolar fricative in the areas where Low German (Plattdeutsch) is spoken such as Hamburg. This sound change also did not occur in other languages. For example, the word "street" and the Dutch equivalent "straat" are not pronounced with an alveolpalatal voiceless fricative.
What is the motivation for the sound change of the consonant clusters "st" and "sp"? The process which converted the alveolar voiceless fricative to a voiceless alveopalatal fricative may be called palatalization. Another possible name for the process is weakening because the voiceless alveopalatal fricative has a lower frequency than the voiceless alveolar fricative. However, the weakening process is more common in syllable-final position than in syllable-initial. This type of weakening occurs in European Portuguese as well as in the Portuguese of certain Brazilian speakers, notably in Rio de Janeiro. These speakers pronounce the syllable-final "s" as a voiceless alveopalatal fricative in words such as "seis" (six), "festa" (party) and "dias" (days).
The best term for this sound change may be dissimilation. The reason is that the alveopalatal voiceless fricative is rather different from the voiceless bilabial plosive and voiceless alveolar plosive. The voiceless alveolar fricative, however, is much closer in place of articulation to the two plosives. They can all be classified as +anterior. The voiceless alveopalatal fricative, however, articulated between the alveolar ridge and the palate, is -anterior. Though palatalization, weakening and dissimilation can all be used to refer to this sound change, I cannot find a clear motivation for palatalization nor weakening. For this reason I prefer to use the term dissimilation.
The Germanic Consonant Shift is associated with many sound changes. Among these is the change of the consonant clusters "st" and "sp"in which the initial consonant changed from a voiceless alveolar fricative to a voiceless alveopalatal fricative. Many terms can be used to describe this change such as palatalization, weakening and dissimilation. However, many linguists prefer to describe this as a process of dissimilation because the motivation for other processes is not easily explained.