the 19th letter of the Greek alphabet
"The professor wrote a large tau on the blackboard to demonstrate how the Greek symbol looks in geometry."
The letter Τ/τ in the Greek, Hebrew and ancient Semitic alphabets, being the nineteenth letter of the Classical and Modern Greek, the twenty-first letter of Old and Ancient Greek.
"The scholar noted that while tau is the nineteenth letter in modern Greek texts, it holds the position of the twenty-first letter in older inscriptions found at archaeological sites."