Sometimes new roots were created in PIE or its early descendants by various processes such as root extensions (adding a sound to the end of an existing root) or metathesis.
When the onset or coda of a root contains a consonant cluster, the consonants in this cluster must be ordered according to their sonority. The vowel constitutes a sonority peak, and the sonority must progressively rise in the onset and progressively fall in the coda.
The rules for the ordering within a cluster of obstruents are somewhat different, and do not fit into the general sonority hierarchy:
Roots were occasionally created anew within PIE or its early descendants. A variety of methods have been observed.
An example of such a pair of roots, both meaning "to increase, to enlarge":