The member deformations can be expressed in terms of the deflections at points $\mathrm{A}$, $\mathrm{B}$, and $\mathrm{C}$. For example, the deformation of member $\left(1\right)$ can be stated as:

${\mathrm{\delta }}_1={\mathrm{u}}_{\mathrm{B}}-{\mathrm{u}}_{\mathrm{A}}$

where ${\mathrm{u}}_{\mathrm{A}}$ and ${\mathrm{u}}_{\mathrm{B}}$ are the deflections of points $\mathrm{A}$ and $\mathrm{B}$ in the $\mathrm{+x}$ direction.

Points where axial members are connected are called joints. In this structure, $\mathrm{A}$, $\mathrm{B}$, and $\mathrm{C}$ are joints. In general, we find deflections at joints and elongations or contractions of axial members.