Wooden buttons tabletop - optimum sizing?

[Rorton]

Is there an optimal size for tabletop buttons?

specifically was thinking about the size of the tongue so I can cut correctly sized slots/grooves, planning on 6mm as I have a 6mm grooving bit for the router, otherwise I guess could cut a complete groove on the table saw, but this will run into the tenons

also the depth of the tongue compared to the slot, should the tongue be about 5mm shorter than the slot?

 

[Sgian Dubh]

Tongue 9 -10 mm long by 6 -8 mm thick. Cut groove to match, i.e., 9 - 10 mm deep by 6 - 8 mm wide, but loose enough for the button tongue to slide in easily. Set groove in the rail ~0.25 - 0.5 mm further in from rail's top edge than the button's thickness, i.e., if the button is 20 mm thick, set the groove in the rail ~20.25+ mm down from the top edge.

In use, particularly on those rails parallel with the top's long grain, the button tongue is only partially inserted into the groove, e.g., a 9 mm long tongue is set approximately 6 - 7 mm deep into the groove. This allows for expansion and contraction of the table top. Buttons at the end of the top's length are inserted fully into the groove because changes in dimension following the longitudinal grain direction are negligible and can generally be ignored, especially in small lengths, e.g., typical coffee table to dining table (and even bigger constructions) dimensions. Slainte.

 

[Rorton]

Brilliant thanks