RUKI Rule
In parts of the Indo-European family, an inherited **s** regularly becomes a retroflex or retracted **sh** after **r, u, k, or i**. That patterned change can explain some cross-language similarities without requiring borrowing.
A Sanskrit sh may descend from an earlier s—not because a speaker copied a new word, but because the preceding sound was r, u, k, or i.
E1The preceding sound does the work
The RUKI Rule describes a conditioned sound change: begin with inherited s, inspect the sound immediately before it, and expect a retracted or retroflex sh in the specified environment. The apparent mismatch between s and sh is therefore systematic rather than accidental. Once the correspondence repeats in the right positions, common inheritance plus regular change becomes an available explanation alongside borrowing.
E1A correspondence is not a verdict
The rule explains only the stated s → sh change after r, u, k, or i. It does not, by itself, establish that every similar word is inherited, rule out borrowing in a particular case, or settle which language is older.
E1Check the sound environment first
When an s/sh resemblance is offered as evidence of borrowing, write the forms side by side and inspect what precedes the disputed sound. If r, u, k, or i appears in the relevant ancestral environment, test regular inheritance before accepting a borrowing story.
E1Episodes that teach this
-
Sanskrit vs Tamil - Which is the Oldest? Future IQ
27,069 views
The episode explains that an Indo-European 's' after r, u, k, or i becomes the Sanskrit retroflex 'sh'.