Underneath the jersey, players usually also wear a tighter fitting undershirt. Undershirts aren't a required part of the uniform, but are quite popular and often seen during games at all levels. Most leagues require undershirts to be a neutral color so as not to distract from the jersey. Long-sleeved undershirts can help keep players warm for games played in colder weather and short-sleeved undershirts are preferred in warmer weather. Compression undershirts can provide extra support for muscles and wick sweat during games, but undershirts can also be as simple as a cotton t-shirt. They're also often worn by themselves in practice.