An armrest structure includes a tubular support, and an armrest having a tubular end telescopingly engaging the tubular support. A support plate engages bottom walls of the tubular support and the tubular end in a laminar arrangement. A clamping bolt slides through a top hole and into aligned holes in the laminar arrangement, with its head abutting the aligned holes. A clamping handle on a bottom of the bolt is rotatable between a clamping position tensioning the bolt to secure the arrangement, and a release position de-tensioning the bolt. By this arrangement, remaining walls of the tubular support and the tubular end are not deformed even when the clamp is clampingly pressing the adjacent sidewall sections together. An adjustable back with actuator handles at a lower edge of the back, and a seat assembly with fingers that retain an upholstery cover tight against concave areas are also shown.