By spacing I wonder if you mean tracking? (the field to the right of the kerning field in InDesign or Illustrator)

Tracking is used for many letters at once (i.e. on whole words like to space up a title made of small caps) and kerning would be to correct the space between a specific pair of letters. At least that's what I do :-)

Em

You kern only letter combinations between which the standard spacing doesn't work well, due to the shapes of the letters.
So for example in most cases HH, HT, HA will be set by the spacing and not require kerning, but TA will require a modification to the spacing, namely, a kerning pair.

In fonts, the thing to investigate is sidebearings.

so wont the overall spacing change.?

Changing the overall spacing is the point. We kern to get an even appearance overall.

hey guys

thanks for the replies. HH, HT doesn't require kerning and i understand that. but TA requires Kerning. so we are changing the space between t and a.Wont this effect the space between H and T. In other words, we change the spacing using side bearings, don't we do the same in kerning? if not what are we changing in kerning? is the change made to spacing by kerning only exclusive to those pair of characters or does that influence other letter pairs also?

Cheers
M

Well, it depends if you are talking about creating a font or using one. Making a font, you can have specific kerning pairs that are exclusive to that pair of characters. This is different than side bearings. If you are setting type, you can kern individual letter pairs and not affect other instances of that letter pair. In setting type you also can have control of tracking which changes the overall horizontal letter spacing of all the selected text.

You need to kern where spacing causes ‘faults’. A well spaced font needs a minimum of kerning.

Here is a decent explanation for the user of digital type, with graphics.

In designing type, yes, kerning numbers only apply to the pairs you specify, whereas the side bearings apply everywhere. The kerning adds or subtracts from the side bearings of the pairs you specify. (Actually I think it's only one of the pair, but that doesn't matter.)

The difference between spacing and kerning is not visual, but something the technology defines.