p,m and b columns in tables

I wonder why p columns are aligned at the bottom and b columns aligned at the top?

created with this code:

\begin{tabular}{|p{0.3\linewidth}|m{0.3\linewidth}|b{0.3\linewidth}|}
\hline
\hline
text which is considerably longer than the width of the column  &
text which is considerably longer than the width of the column  &
text which is considerably longer than the width of the column
\tabularnewline
\hline
\end{tabular}

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Possible duplicate of tex.stackexchange.com/questions/33257/… It is the expected result. –  egreg Nov 18 '11 at 14:42

• p means normal cells, they are like parbox with alignment at the top line
• b means alignment at the bottom, so the baseline is at the bottom line
• m means alignment in the vertical center, i.e. the baseline is in the center.