Another wood movement question

[jmack77]

So, I'm building a display shelf (it's really nothing more than a bookcase) for a store.  The shop owner has requested that for the back I use horizontal wood planks.  So, I planned down some poplar that I had lying around from three quarter inch to half inch.  Now my question is this.  Should I just butt joint the planks, or put a rabbit on each plank with a slight gap to account for wood movement?  I will only be nailing the planks to the case sides and a divider.  I am in the eastern panhandle of WV, and it has been very cool and dry for the past week or two.  The poplar has been sitting in my garage for about 6 months.  

[wtnhighlander]

I would use a ship-lap joint (rabbets on each edge, but facing opposite directions) with a chamfered face edge to create a shadow line. Unless your boards are really wide, movement shouldn't be a huge problem, but the overlapping rabbets keep any light from showing through if the woods does shrink, and the shadow line makes it even harder to notice a gap. Just leave some expansion room where the back meets the top and bottom of the case.

[wdwerker]

+1 on shiplap rabbets w a Chamfer. I use a laminate sample chip to space the boards just in case they swell a bit after assembly. When you start with a 1/32 to 1/16 gap at each joint plus the chamfers any future movement either way is allowed for.

[Byrdie]

Agree with shiplaps - it forms a cleaner joint - won't show a gap if there's any wood movement.