Orientation of a LED is important since a LED will not work if its oriented wrong.

 

If we first look at normal LED in the picture bellow then you can see that you have two ways to identify the + and – or anode vs cathode. Its the length of the pins where the longer wire is the anode and the shorter is the cathode. If wires have been trimmed then you can usually also see it on the LED it self the flat side on the edge is the cathode or – as is shown on the picture to right here.

For RGB led then things get a bit more complicated. RGB leds usually share common cathode (black which would be the longest wire or you can see it inside the glass by the shape there if the wire has been shortened), and then three anodes, red on the left then greed right of the cathode and then blue on far right.

(To complicate it a bit then I have seen RGB led where the anode was shared and there were three cathodes)

For surface mounted LEDs then it can vary greatly. Here are some common ways to mark the Cathode on surface mounted LED’s:

If you cannot find cathode marker on your surface mounted LED then you may have to look up its data-sheet.

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