\fontencoding {<encoding>}
\fontfamily {<family>}
\fontseries {<series>}
\fontshape {<shape>}
\fontsize {<size>} {<baselineskip>}
\linespread {<factor>}
Each of the commands starting with \font... sets one of the font
attributes; \fontsize also sets \baselineskip. The \linespread
command prepares to multiply the current (or newly defined)
\baselineskip with <factor> (e.g., spreads the lines apart for
values greater one).
The actual font in use is not altered by these commands, but the
current attributes are used to determine which font and baseline skip
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 six commands above, 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 (or \linespread) and a \selectfont.
\usefont {<encoding>} {<family>} {<series>} {<shape>}
A short hand for the equivalent \font... commands followed by a
call to \selectfont.