
This is a flat slot, but like most flat slots it has a slight curve on one side. Don't assume that all flat slots are the same size. It is not. This spline is closer to 5/16 in width, but it worked great to replace an old 1/4" spline that split after 25 years. I couldn't find a 1/4" (0.025) anywhere. Find Spline so I took a chance and it works fine. It is black or very dark brown. It's very flexible. The old things were as hard as bricks. Watch some installation videos and it should be easy enough.

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