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Brahmic scripts such as devanāgarī, Gurmukhī, and Bengali are noted for joining glyphs with a long horizontal line across the top (shirorekhā). However, other examples such as Tamil use more curls and curves, and lack the shirorekhā.

I am told that one reason for this change was because manuscripts in certain regions used parchment or vellum, and in other regions used palm leaves. Writing on a palm leaf, if you made long, straight horizontal marks on it, you could split the fibers.

https://commons.wikimedia.org/wiki/File:Thiruvaymoli_palm_ma...

https://commons.wikimedia.org/wiki/File:Palm-leaf_manuscript...