Long grain means that the grain runs parallel with the long sides of the sheet. All normal retail paper is long-grain. Short grain ~ or cross grain ~ has the grain running parallel with the shorter sides.
Bookbinders will always arrange their work so that the grain runs parallel with the hinge. Bookcrafters may find it too troublesome ~ for various reasons ~ to do that.
Firstly ~ paper expands when wet ~ then it shrinks again as it dries. The greater expansion is across the grain [127278].
Secondly ~ paper curls (if it can) when damp ~ and may not uncurl when it dries! [124341]
Thirdly ~ paper folds better with the grain.
and ~ availability ~ Some sellers will only stock stuff that has a rapid turnover.
and ~ cost ~ Most papers are cut in the mill as long grain.Short grain papers require additional cutting by someone.
and ~ storage ~ Sellers may not have space to store supplies of both types of paper.
and ~ confusion ~ clear labelling of all those odd piles of left over paper is required.
I shall expand on these paragraphs, with illustrations ~ when I get time ~ in 2022 perhaps.
Craftworkers need to know about grain. Bookbinders do know all about it ~ it is one of the reasons why their training requires years of experience. Woodworkers learn ~ and respect ~ and allow ~ for grain. Litho printers need to know all about it. Engineers machine operators need to allow for it in the design of printing machinery.
~ Ways of finding the direction of paper's grain (2) wet it ~ [122092]
~ Ways of finding the direction of paper's grain (3) ~ cheat [123382].
~ Ways of finding the direction of paper's grain (4) ~ paste it [124341].
~ Ways of not finding the direction of paper's grain (5) ~ guess it [117509]