Tongue and groove is the traditional method to fit flooring. The planks of flooring have one long side and one short side with a tongue (machined protruding edge) and one long side and one short side with a groove (machined rebated edge). When fitted together the tongue should fit into the groove perfectly creating smooth, gapless flooring. The floor can either be secret nailed, secret screwed or glued directly down to the subfloor, or can be floated over an underlay by gluing along the length and width of the plank and pushing them together.
Tongue and groove flooring is available in both soild and engineered wood, so can be fitted on any subfloor that has been well prepared.
The down side of tongue and groove flooring is that it can be quite challenging to install and may need a professional wooden floor fitter to ensure the floor is correctly installed and to obtain minimum wastage.