The line separating solids and liquids can slope either to the left ( negative ) or to the right ( positive ). In most cases, the slope is very small. When the slope is negative, an increase in pressure decreases the melting point. When the slope is positive, an increase in pressure increases the melting point.

This also affects the density of the solid versus the liquid phase. When the slope is negative ( to the left ), the liquid phase is more dense than the solid phase. This allows the solid to float on the top ( ice and water ). Most materials have a positive slope ( to the right ) and the solid is more dense than the liquid. This has the solid sink to the bottom. The phase that is higher on the vertical line, is the more dense.

Observe that the triple point can be either lower or higher than the normal melting point. This depends on the slope of the solid/ liquid line. Of course, the triple point can also be at a pressure higher than one atmosphere.