The low-level commands used to select a text font are as follows.
\fontencoding {<encoding>}
\fontfamily {<family>}
\fontseries {<series>}
\fontshape {<shape>}
\fontsize {<size>} {<baselineskip>}
Each of these commands sets one of the font
attributes; \fontsize also sets \baselineskip. The
actual font in use is not altered by these commands, but the current
attributes are used to determine which font to use after the next
\selectfont command.
\selectfont
Selects a text font, based on the current values of the font attributes.
Warning: There must be a \selectfont command
immediately after any settings of the font parameters by (some of)
the five \font<parameter> commands, before any following text.
For example, it is legal to say:
\fontfamily{ptm}\fontseries{b}\selectfont Some text.
but it is not legal to say:
\fontfamily{ptm} Some \fontseries{b}\selectfont text.
You may get unexpected results if you put text between a
\font<parameter> command and a \selectfont.
\usefont {<encoding>} {<family>} {<series>} {<shape>}
A short hand for the equivalent \font... commands followed by a
call to \selectfont.