Sheer or blockout curtains?
Start with when you use the room. Sheers soften daylight while you enjoy the view. Blockout curtains suit bedrooms where morning light is unwelcome. You can also put one of each on a double track and draw them separately.
Sheer curtains
Soft folds that filter the daylight.
Blockout curtains
Full folds in a coated or lined fabric for privacy after dark.
Which curtain suits your window?
For a daytime view
Try a sheer if you want to soften the view from the street while keeping some outlook. Compare the weave from your usual seat. With the lights on after dark, people outside may still see in.
For sleep or evening privacy
Choose blockout curtains if you want an opaque fabric across the glass. For an east-facing bedroom, pay attention to the top of the track and the edges where morning sun can slip past.
For the same room, day and night
A double track gives the sheer and blockout curtain their own tracks. Leave the sheer drawn for daylight, close the blockout for privacy, or draw both aside for a clear view. At a sliding door, check where both sets of folds will sit.