In case you don"t make a claim in the current block
In case you don"t avail of the LTA exemption in a particular block, whether for both the journeys or for one journey, then you could carry forward one journey to the first calendar year in the next succeeding block of four years. Thus, in the next block of four years, you could claim the carried forward travel, plus, two journeys of that particular block i.e. a total of 3 exemptions!

Let us suppose you are entitled to an LTA of Rs.10, 000 per year and you do not utilize it in the block 2010-2013. This amount could be carried forward in the next block of four years. You must thus claim this amount in 2014 and the LTA entitlement of that particular block (i.e. 10,000+10,000) could subsequently be claimed according to the specification of LTA laws as mentioned above.

When Husband and Wife both receive LTA
In such a case, both of them could claim LTA individually as the rules of LTA apply individually to each of them. So in a block of four years, each spouse can claim LTA twice. The only restriction is that both spouses cannot claim an LTA exemption for the same journey. In other words, LTA cannot be claimed twice for the same journey.

So How Does One Claim an Exemption?
Thought there is no statutory obligation for employers to collect evidence or bills for the travel, it is better to keep them as a record. You would be required to submit a written declaration of the amount utilized for travelling to your company. In case you encash your LTA without actually performing any journey, the entire amount would be added to your salary and taxed according to relevant income tax rates. Also you could claim exemption only to the extent of your LTA eligibility. For example, if you are eligible for an annual LTA of Rs 20,000, you can claim exemption only up to that amount. You cannot claim exemption of Rs 50,000 even if you have actually spent that much. Suppose you have spent Rs.18, 000 on travel, then only that amount gets an exemption and the balance 2,000 would be taxable.

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