Also, something that isn't readily apparent to everyone is that although leading may seem like a paragraph attribute, it is a character-level setting in InDesign. Notice the gap between lines in this screen shot: The leading is set the same for all of the selected text, and there is no space-before or space-after involved. If I select all of the subsequent lines, it shows the same thing: But, what isn't shown is that I didn't select the paragraph return at the end of the line with the space above: …which shows where the extra leading is coming from. Probably the most common reason why people post about unexplained line spacing is because of the settings of a paragraph return. And to your question about baseline grids, yes, you will need to set the grid to the appropriate setting if you want to lock text to it. In the same text I used in the above examples, notice how the gap is no longer visible when I snap the guides to the default 12-pt baseline in this screen shot: Because the leading is set to auto (14.4 for 12-pt type), it won't fit within the 12 points that separate the baseline grid, so it skips to the next available line. And because the extra leading on the one paragraph return is set to 22, it isn't enough to push it two lines down. That would need 24 or better. So if you want to snap your lines to the grid, set the grid to what ever you choose (1x, 1.5x or 2x). If you use space-before or space-after amounts that are less than the leading, you won't get double spaces between paragraphs.
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