def most_frequent(s):
    """
    Count the ocurrence of letters in the given string (s) and return a list of tuples
    in descending order of frequency.
    """
    freq = dict()
    for c in s:
        c = c.lower()
        if c >= 'a' and c <= 'z':
            freq[c] = freq.setdefault(c, 0) + 1
    res = []
    for ltr, cnt in freq.items():
        res.append((cnt, ltr))
    res.sort(reverse=True)
    return res