Download Arduino Uno Sda Scl PNG. And the documentation says a4 and a5 pins can also be sda/scl. Pin19 (no label on the pcb front, only visible from the side).

Raspberry Pi and Arduino Connected Using I2C - Oscar Liang from oscarliang.com

The arduino due has two i2c / twi interfaces sda1 and scl1 are near to the aref pin and the additional one is on pins 20 and 21. And the documentation says a4 and a5 pins can also be sda/scl. The 2 first pins side to usb connector are sda/scl according to documentation.

The scl line is the clock signal which synchronize the data transfer between the devices on the i2c bus and it's generated by the master device.

The arduino due has two i2c / twi interfaces sda1 and scl1 are near to the aref pin and the additional one is on pins 20 and 21. I am not able to find this info anywhere but regarding it is a i2c protocol it does make sense to be a ttl technology. After the data is completely sent, the transfer will end with a stop condition which occurs when the sda line goes from low to high while the scl line is high. Scl is the clock signal, and sda is the data signal.