What size nail do I buy so it can fit into this honeycomb bong from smokeday.com http://www.smokeday.com/10-tilted-honeycomb-water-pipe.html That's the bong I'm buying and it says 19/18mm joint and so do I buy a 19/18 mm nail or a 18 mm nail and does it have to be a male or female because I don't know anything about joint sizes and how they work. I found this nail will it fit onto the bong? http://www.smokeday.com/domeless-titanium-nail-fits-18mm-or-19mm-pipes-includes-ceramic-adapter.html The bongs joint is 19/18 while the nail is 18/19 so wouldn't it have to also be 19/18 for it to fit?
Assuming the bong you have/are getting is the one in the picture, all you need is an 18mm male nail. Depending on if the nail is domeless or not you will need to purchase a dome.