After all the preaching about using the keyboard more than the mouse in Microsoft Word, I'm now advertising some quick selection tips for the mouse. Still, these tidbits are nice to know when you need the mouse. Plus, it’s good to have the basics down before moving on to more and better methods.
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Less common mouse selections
We've all used the mouse literally thousands of times, but you probably haven't used at least some of the mouse selection tidbits below. Hopefully, you'll discover (or be reminded of) some little editing gems.
We further give some keyboard shortcut alternatives where applicable. Some of them are standard word shortcuts, but others utilize custom macros or Word commands which can also be assigned to shortcuts as needed. A few tips work either in Word for Windows or Mac, but options are given to enable similar functionality on the other system if desired.
Use the margin to select content
We can select content with the mouse using the margin to either side of the main text.
Click once (or drag) in the margin
Clicking once in the margin near some text selects the line nearest to the mouse cursor. Similarly, drag up or down in the margin extends the selection up or down from the starting line.

I've used this a lot since it cleanly selects whole lines without needing to precisely drag across the precise text start to end. The keyboard equivalent is Shift+Up or Down arrow, but these shortcuts select lines from the starting position, but a quick Home key tap before the selection will make them work the same.
Double click in the margin
Double-clicking in the margin selects the whole paragraph.

This one is also useful, but several of my personal editing macros make it much less commonly used in practice. A previous article includes several such macros for working with whole paragraphs, but see the SelectParagraph variation to select the current paragraph.
Triple click in the margin
Triple clicking in the margin selects the whole document.
This document selection is nice for me because I often override the standard Word keyboard shortcut for it. On the other hand, if you haven't overridden like I do, the Control+A shortcut in Word for Windows (or Command+A on a Mac) does the same thing only faster.
Click a sentence
Command (or Control in Windows) clicking anywhere within a sentence selects the whole sentence.

This tip is convenient, but I always forget it. A previous article also covers various sentence manipulation macros (see the SelectSentence variation) keeping your fingers on the keyboard more.
Click part of a sentence (Mac)
Similarly Command+Option click on a sentence when using Word for Mac selects from the beginning up to the mouse click position in the sentence. The corresponding Control+Alt click is not assigned in Word for Windows, but another article covers how to assign shortcuts to two standard (but kind of hidden) Word commands that also select partial sentences with no macros involved.
Unfortunately, if you accidentally start dragging when you click, it will start a block selection. This tip also doesn’t play nice with creating non-contiguous selections since they both use the Command modifier key.
Drag headings in the Navigation Pane (Windows)
In Word for Windows, we can drag headings and all their content up or down in the document using the Navigation Pane.
It doesn't work perfectly since the subheading will sometimes improperly pick up the heading style where it is moved, but it's still nice in longer documents. I find it handy for reorganizing novel notes, but I also use it in my novels since I outline in real time as I write (which is a messy back and forth process for me).
This feature isn't implemented in Word for Mac, so you’ll need to use the inferior Outline View to move headings around in a similar way which is annoying. If you prefer to directly manipulate headings, a separate article covers a macro to select the current heading and another member article extends that capability some.
More common mouse tips
You may already know some or all of these, but I included them for completeness.
Shift click
Holding Shift and clicking once in the document will select (or extend the current selection) all the content from the initial insertion point (or selection) to the clicked position.

Word seems to like to extend over the entirety of the first word but not the last, but this behavior is likely linked to the selection settings in Word preferences.
I like this tip, but I usually forget it and just drag across the text. It is definitely useful for selections that span a page boundary. A shortcut variation is to press F8 to enable extended text selection and then click where you want to end the selection.
Double click a word
Double-click a word to select it. I use this one a lot.
Double click part of a word (Mac)
Option double click on a Mac will select text from the beginning up to the click position in a word, but the feature is not enabled in Word for Windows. Enabling this feature in Word for Windows would only take a one or two line macro. A quick tweak of the steps in another article for selecting partial sentences would also work with no macros involved. Alas, no current article explicitly covers it.
Triple click a paragraph
Triple-click inside a paragraph to select the entire paragraph. This seems redundant with just regular double clicking in the margin, but it's there if you like it. A previous article includes a SelectParagraph macro in addition to several other variations.
Drag selections
We can drag an existing selection with the mouse to a new location in the document. I use it in both Word for Windows and Mac, but I find it annoying on a Mac because Word creates a hovering thumbnail of the moved content. It's translucent, but it still blocks the view if I'm dragging the text anywhere nearby the original position.
Non-contiguous selections
Hold Command on Mac or Control on Windows to select other text in Word even when the selections are not contiguous.

Anything action works on each separate selection saving you the effort of reapplying the action separately to each block of text. This is a nice feature, but I don't use it often in practice. In testing, it also conflicts with some of the other mouse selection tips above as if Word enters a different "mode" of some sort for some of the enhanced selection techniques.
Block selections
Holding down Option on Mac or Alt on Windows block selects content making a vertically connected series of non-contiguous selections.

It’s a special case of non-contiguous selections that is quite useful when you need it. At first, I thought I’d never use this feature, but I encountered a situation editing some timeline text on my YouTube channel where it was the perfect solution. I selected all the description text to the right of the times and deleted all the descriptions same time.
Block selection from the start of the sentence
Holding down Command+Option (or Control+Alt on Windows) and dragging automatically creates the block selection from the beginning of the current sentence. It can feel different depending on the relative positions, but it's always from the start of the sentence whether that is to the right or left of the initial mouse cursor position.
It's a specialized case of a specialized tool, so it probably won't be used often, but your mileage may vary.
Keyboard or the mouse?
Using the keyboard is still faster for many (maybe most?) editing situations, but the mouse is better for some selections. No one says we can only have a hammer in our toolbox, so use both as appropriate. Just don't get lazy and always reach for the mouse.