Britax Safe'n'Sound Instructions For Installation & Use page 34

HOW TO USE
D. Place each arm through each shoulder strap of the harness.
Position the buckle of the crotch strap as shown right.
E. Fit either harness buckle tongues into the harness buckle
followed by the other tongue. A click is heard when the last
tongue is fully engaged.
Should the second tongue not insert - press the red button
and reinsert the tongues. Ensure secure engagement by
pulling harness straps. If the harness buckle does not engage
correctly then it may require cleaning or need replacement
- refer to page 43. The buckle tongues are fully engaged
when both tongues are inserted and 'click' is heard.
Tongue
34
Instructions for Installation & Use
Buckle
Click