How do you know when there will be a sheath?

[escualida]

Hi everyone, this might seem like a silly question but I've only been growing orchids for a short time and in that time I have never had a new lead form a sheath. My question is, when a new lead is growing, at what point do you know if it will have a sheath? I think I read that it develops at the same time as the lead so if the new pbulb reaches full height and doesn't have a sheath does that mean it won't grow one?

Thanks in advance!