The crease used to be a rectangle. After a while, a larger semicircle crease was added around the original. Eventually, the interior rectangle was removed, and L-shaped markings were left to indicate where the previous crease dimensions used to be. When the NHL reduced the semicircle crease to its current domed-rectangle shape to cut down on crease violations, the L-shaped markings became the lines you see today.
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The crease used to be a rectangle. After a while, a larger semicircle crease was added around the original. Eventually, the interior rectangle was removed, and L-shaped markings were left to indicate where the previous crease dimensions used to be. When the NHL reduced the semicircle crease to its current domed-rectangle shape to cut down on crease violations, the L-shaped markings became the lines you see today.
[Source](https://www.thefaceoff.net/history-of-the-rink)