これは、以下のWWWページで紹介されていました。
[Tip] Use “Win+Shift+S” Hotkey to Take Selected Area Screenshot in Windows 10文章も引用します。
http://www.askvg.com/tip-use-winshifts-hotkey-to-take-selected-area-screenshot-in-windows-10/
You just need to press "Win+Shift+S" keys together and your computer screen will be covered by a white overlay. Now you can select any desired area of the screen using your mouse cursor. As soon as you release the cursor, the selected screen area screenshot will be automatically copied to clipboard.MacOS Xでは”Shift+Command+4"でできます。
Mac でスクリーンショットを撮る方法
https://support.apple.com/ja-jp/HT201361