Between Ancient Greek and Modern Greek, a sound change affected the consonant λ l when it occurred before another consonant; in particular, the λ l became ρ r. Thus, Ancient Greek ἀδελφός adelphós ‘brother’ became ἀδερφός aðerfós. This change can be called “delateralization” because λ l is a lateral consonant (air released along the sides of the tongue) whereas ρ r is not. This example will be useful to students of Modern Greek because of the reintroduction of the ancient form; even beginning students will inevitably ask for an explanation of the co-occurrence of these forms, as of the comparable καλώς ήλθατε/ήλθες for καλώς ήρθατε & ήρθες.
This change explains such forms as αρβανίτικα ‘the Albanian dialects spoken in Greece’, with –r-, vs. αλβανικά ‘the Albanian language (spoken in Albania)’, where the form with -l- reflects the etymological source and the form with -r- reflects the effects of the delateralization change, or ήλθες vs. ήρθες for ‘you came’. Moreover, pairs such as these point to the importance of recognizing the katharevousa vs. dimotiki distinction (so that the distinction between ήλθες and ήρθες is to be understood as a katharevousa vs. dimotiki difference, with the dimotiki form reflecting the effects of the delateralization change.
In the case of αρβανίτικα vs. αλβανικά, the difference reflects the delateralization but also the fact that a borrowed word (αλβανικά coming from the Latin) does not (necessarily) undergo the change (note too μπαλκόνι balkóni ‘balcony’, with -l-, not **barkóni(!), a borrowing from Italian).
At the risk of making things seem complicated, but by way of showing how various changes can interact with one another, we can make sense of the change (resulting in the katharevousa vs. dimotiki difference) between the form of the name Στέλιος stelios and the form Στἐργιος steryos: we know from the i-to-y page that i turned into y before another vowel so that stelios would turn into stelyos; at that point, the delateralization change comes into play because now the –l– stands before a consonant (which it did not prior to the i-to-y change), so that steryos would result (spelled Στἐργιος).