The invention relates to a lock and key combination including a cylinder lock with a rotatable key plug having a key slot for receiving a flat key blade (100) on a key, and at least two side locking tumblers (201a, 201b; 301, 302; 401, 402) which are guided in associated chambers in the key plug and have transverse fingers ( 211a, 2 11b : 311, 312; 411, 412a, 412b) projecting into the key slot. There is a code pattern (110) in a side recess (115,118) of the key blade comprising at least two separate code surfaces formed on at least one side surface (101) of the key, including a primary code surface (131- 135; 141''; 151) located in an external portion of the side recess and adjoining said one side surface (101), and a secondary code surface (141-145; 142 ' 152 ) being defined in a groove forming a deeper portion of the side recess. The primary and secondary code surfaces engage with first and second fingers (211b, 311, 411 and 211a, 312, 412a) on associated side locking tumblers. Master key systems can be designed by using such locks and keys.