The memoir class - The UK TeX Archive

E.2. LaTeX filesto get the values of the arguments, and then the macro name is replaced by the definition.The expansion processor then looks at the first token in the replacement, and if thatis expandible it expands that, and so on.Nominally, the eventual output from the expansion processor is a stream of nonexpandibletokens. There are ways, however of controlling whether or not the expansionprocessor will actually expand an expandible token, and to control the order in whichthings get expanded, but that is where things get rapidly complicated.The layout processor works something like this. Ignoring maths, TeX stores what youtype in two kinds of lists, vertical and horizontal. As it reads your words it puts them oneafter another in a horizontal list. At the end of a paragraph it stops the horizontal list andadds it to the vertical list. At the beginning of the next paragraph it starts a new horizontallist and adds the paragraph’s words to it. And so on. This results in a vertical list ofhorizontal lists of words, where each horizontal list contains the words of a paragraph.It then goes through each horizontal list in turn, breaking it up into shorter horizontallists, one for each line in the paragraph. These are put into another vertical list, so conceptuallythere is a vertical list of paragraphs, and each paragraph is a vertical list of lines,and each line is a horizontal list of words, or alternatively one vertical list of lines. Lastlyit chops up the vertical list of lines into page sized chunks and outputs them a page at atime.TeX is designed to handle arbitrary sized inserts, like those for maths, tables, sectionaldivisions and so forth, in an elegant manner. It does this by allowing vertical spaces on apage to stretch and shrink a little so that the actual height of the typeblock is constant. Ifa page consists only of text with no widow or orphan then the vertical spacing is regular,otherwise it is likely to vary to some extent. Generally speaking, TeX is not designed totypeset on a fixed grid, but against this other systems are not designed to produce highquality typeset mathematics. Attempts have been made to tweak LaTeX to typeset on afixed grid but as far as I know nobody has been completely successful.TeX works somewhat more efficiently than I have described. Instead of reading thewhole document before breaking paragraphs into lines, it does the line breaking at the endof each paragraph. After each paragraph it checks to see if it has enough material for apage, and outputs a page whenever it is full. However, TeX is also a bit lazy. Once it hasbroken a paragraph into lines it never looks at the paragraph again, except perhaps to splitit at a page break. If you want to change, say, the width of the typeblock on a particularpage, any paragraph that spills over from a previous page will not be reset to match thenew measure. This asynchronous page breaking also has an unfortunate effect if you aretrying to put a note in say, the outside margin, as the outside is unknown until after theparagraph has been set, and so the note may end up in the wrong margin.E.2 LATEX FILESThe aux file is the way LaTeX transfers information from one run to the next and theprocess works roughly like this.• The aux file is read at the start of the document environment. If \nofiles hasnot been specified a new empty aux file is then created which has the side effect ofdestroying the original aux file.431