I hope this makes sense..

At around 5 vdd.

A TTL input pin will go "HIGH" around 1.1 volts. 1.1 volts is the ON?OFF threshold.

A Schmitt input will go "HIGH" around 3 volts but stay high until it drops below 1.6 or so.

Your meter may vary a bit. Mine does

Personally in an application I will not call an input high before 3.6 volts and LOW has to be 1.5 or less. This is dealing with limit switches and such. Basically I figure it is either on or off, Dirty contacts, bouncy switches and such adds to the problem.

If I need to split hairs (volts) I do not like to rely on TTL or ST for the threshold. I set up an ADC.