The difference between the Spanish letters "ere" (r) and "erre" (rr) lies primarily in their pronunciation:
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"ere" (r) - This is usually a single, soft "r" sound, which is pronounced like the English "r" in "butter" or "ladder" when it occurs between vowels. At the beginning of a word or after "l," "s," or "n," it is pronounced as a slightly stronger sound.
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"erre" (rr) - This represents a rolled or trilled "r." It involves a longer, more pronounced vibration of the tongue against the roof of the mouth.
So, the main difference is that "r" is a single tap or flap of the tongue, while "rr" is a rolled sound that lasts longer.