What letter should appear next in this sequence z s m h d?

The sequence z, s, m, h, d consists of letters that correspond to decreasing intervals of 3 in the alphabet. Starting with z (26), the letters are s (19), m (13), h (8), and d (4). The next letter should follow this pattern, which means subtracting 3 from d (4), leading to the letter 'a' (1). Thus, the next letter in the sequence is 'a'.