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Wake up, the letter is z, that is ▬ ▬ ● ● in Morse code.
A mnemonic is to imagine z with the top and diagonal bars bolded, and two dots on the bottom bar, with one on its left end and the other on its right end. The z straightens horizontally as the top bar moves to the left and the bottom bar moves to the right, carrying the two dots. The first two bars separate as the third fades, forming ▬ ▬ ● ●.
Here is the practice: