rei- / Indo-European roots
Examples of words with the root rei-: derive, Ember Day, rennet, rill, rille, rival, rivulet, run, runnel.
To flow, run.
1. Suffixed zero-grade form *ri-nu-.
a. run, runnel from Old English rinnan, to run, and Old Norse rinna, to run (from Germanic *rinnan, to run, from *ri-nw-an), and from Old English causative ærnan, eornan, to run (from secondary Germanic causative *rannjan);
b. Ember Day from Old English ryne, a running, from secondary Germanic derivative *runiz;
c. rennet from Old English *rynet, from secondary Germanic derivative *runita-.
2. Suffixed zero-grade form *ri-l-. rill from Dutch ril or Low German rille, running stream, from Germanic *ril-.
3. Suffixed form *rei-wo-. rival, rivulet; derive from Latin rīvus, stream.
[Pokorny 3. er- 326.]