der Stuhl
chair
A piece of furniture for one person with a seat, legs, and usually a backrest.
Wir brauchen noch einen Stuhl, weil ein Gast mehr kommt.
We still need one more chair because one extra guest is coming.

chair / stool
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chair
A piece of furniture for one person with a seat, legs, and usually a backrest.
Wir brauchen noch einen Stuhl, weil ein Gast mehr kommt.
We still need one more chair because one extra guest is coming.

The meaning 'Kot' (feces) for 'Stuhl' (stool) is used almost exclusively in a medical context, for example at the doctor's office or in a hospital. In everyday situations, using it can lead to embarrassing misunderstandings.
Für die Untersuchung ist eine Stuhlprobe erforderlich.
A stool sample is required for the examination.
As a piece of furniture, 'Stuhl' (chair) is often combined with verbs of position or movement. Typical phrases are 'auf einem Stuhl sitzen' (to sit on a chair), 'sich auf einen Stuhl setzen' (to sit down on a chair), or 'einen Stuhl an den Tisch rücken' (to move a chair up to the table).
Kannst du bitte noch einen Stuhl aus dem Keller holen?
Could you please get one more chair from the basement?
The idiom 'zwischen zwei Stühlen sitzen' (to sit between two stools) is fixed and describes a difficult decision-making situation or a conflict of loyalty. The structure always stays the same.
Als Manager muss er oft zwischen den Interessen der Mitarbeiter und der Firmenleitung entscheiden und sitzt dabei zwischen zwei Stühlen.
As a manager, he often has to decide between the interests of the employees and the company leadership, and in doing so he sits between two stools.
'Stuhl' (chair) is the base word for many types of seating furniture. Examples include 'Bürostuhl' (office chair), 'Rollstuhl' (wheelchair), 'Liegestuhl' (deck chair), or 'Kinderstuhl' (child's chair/high chair).
Mein neuer Bürostuhl ist viel bequemer als der alte.
My new office chair is much more comfortable than the old one.
bank / bench
'die Bank' and 'der Stuhl' can both be pieces of seating furniture. The most important difference is their size and typical use. Several people can usually sit on a 'Bank' (bench), often outdoors in a park, in a garden, or in public spaces. A 'Stuhl' (chair) is normally for one person and is often placed at a table, in an office, or at home. The words can be confused because you sit on both of them. If you want to emphasize that it is a single seat for one person, 'Stuhl' fits better. If several people can sit next to each other, 'Bank' is more natural.

Am Esstisch fehlt noch ein Stuhl für den Gast.
There is still a chair missing at the dining table for the guest.
Nach dem Spaziergang setzen wir uns auf eine Bank im Park.
After the walk, we sit down on a bench in the park.