Difference between one glue and two glues after each other

Question

Is there a difference between

A\hskip 1pt \hskip 0pt plus 1fil B

and

A\hskip 1pt plus 1fil B

?

Are these two glues the same in terms of spacing and (line-)breaking? Or do they just add up, as I assume?

Answer 1

For spacing and line breaking rules, two glue nodes are exactly the same as one glue except that the algorithms run a little slower. So, the distinction only matters if

• you do node processing with \unskip (or using luatex)
• when the absolute of one of the combined parts would be larger than \maxdimen (in which case you cannot specify the single glue without an error)
• when you parse the output of \showbox somehow.

Answer 2

They are mostly the same. The difference comes if you put a \unskip before B. In the first case, there will be 1pt of space between the A and B. In the second case, there will be no space.