Abstract: An extension level apparatus suitable for use with a carpenter's level includes in combination a pair of elongated frame members adapted to receive therein the carpenter's level and also includes a spring loaded extension member which permits the extension level to be retained in a wall opening into which a door is to be inserted. The extension level permits the user thereof to determine rapidly the exact vertical position of one vertical edge of the door jamb, allowing for shimming where necessary, and permits a measurement to be taken from a starting point to an end point where the second edge of the door opening is to occur. Hereagain the vertical alignment may be rapidly obtained with the use of shimming when necessary, thereby permitting the door jambs to be inserted rapidly and with accuracy.