The barrel is a unit of volume used in the oil industry with the symbol bbl.
Oil is more commonly expressed in terms of cubic metres (m³) but in the US and in the economic world (mainly because such a large percentage of the world's oil is shipped to the US) as barrels.
The measurement of the barrel originated in the US at the first oil well in Pennsylvania. There were so many variants of barrel size (including those used for wine and other goods) that the distrust became so prevalent a central and standard barrel size was created. This was finally established in 1872 as 42 US gallons.
The peck is a unit of volume in the US customary unit system. It uses the symbol pk.
1 US peck is equal to 1/4 of a US bushel. This is also equal to 2 US dry gallons, 8 US dry quarts or 16 US dry pints. In SI / metric, this equates to 8.809768 L.
Although the imperial peck is almost obsolete, it survives in some places in the USA - where some products (namely apples) are still sold by the peck.
It appears in the old English nursery rhyme; 'Peter Piper' where he 'picked a peck of pickled peppers'.