Tongue and groove

[Ali500]

This is probably a daft question in this image of wetwall panels what one is the tongue

 

[noseall]

This is probably a daft question in this image of wetwall panels what one is the tongue View attachment 388487

The one on the left.

 

[johnny2007]

Left

 

[blup]

This is probably a daft question in this image of wetwall panels what one is the tongue View attachment 388487

No such thing as a daft question in GD (left)

 

[accelerator]

As above, it's the left because that has the stick out bit, and the piece on the right has the matching groove (or recess if you like) for it.