Gündüz Alp was the son of Ertuğrul and the brother of Osman I, the founder of Ottoman Dynasty.According to some sources, the name of father of Ertuğrul was also Gündüz Alp and thus the grandfather of Osman I.Recently,[when?] three coins minted by Osman I in Yenişehir-Bursa during his reign which reads "Osman bin Ertuğrul bin Gündüz Alp" proved that the name of the father of Ertuğrul was Gündüz.In conclusion, many contemporary history professors like İlber Ortaylı, Halil İnalcık, Erhan Afyoncu, Yılmaz Öztun.and Osman Turandemonstrated that those history books written more than six hundred years ago had erred in determining the father of Ertuğrul for some reasons, and had given Ertuğrul's father name incorrectly as Suleyman Shah instead of Gündüz Alp because of the similarity of the name of Kutalmışoğlu Suleyman Shah the founder of Seljuks of Rum, and the legendery ancestor of Osman Gazi namely Süleyman Şah ibn Kaya Alp.
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