T-tunics are known as early period garb - correct at least right through the 1200s. A t-tunic is correct garb for most parts of Europe throughout this period.
What is this? This is The Garbindex - a guide to making garb for recreationists (ie how to sew clothes that will make you look genuinely medieval, when 'playing' with such groups as the SCA, doing LARP - or even making something really genuine for drama.)
Use these resources to find resources that will help you create clothing that looks great - and real!