I have this breadboard, and there is no indication on it about what is positive/negative on it.
In comparison, this other breadboard (pic from the internet) DOES have indications:
Can anyone give me a hint ?
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You decide which rails are positive and which are negative when you connect them to the + an - from your power supply or battery.
A breadboard isn't polarised per se. It's just a bunch of cross-connections inside a perforated plastic case. No active or passive components inside.
Some manufacturers may choose to paint indications regarding where to connect the positive and negative rails, but those are just a guideline. Ultimately it's your choice, you're free to connect + and - wherever you want in the breadboard rails.
As gre_gor said. A simple breadboard contains of rows and columns of connections but sometimes for convenience manufacturer marks or prints Positive and Negative symbol along the row as they are usually used to power a circuit
Connections in the two strips along the edge running top to bottom in your image are connected (in the way you have the image placed) top to bottom. So, all pins are connected to each other in each of those rows. Within the inner secions, the connections are (in your image) across from one side to the other.
You decide how you want the power supply cable to be connected to your breadboard, just becasue there's no indicators means that they simply haven't drawn any coloured lines on the breadboard. You can draw red and black lines on however you wish.